test: add cmdtest package (#15251)

## Description

This PR introduces the `cmdtest` package, offering a lightweight wrapper around cobra commands to simplify testing CLI utilities.

I backfilled tests for the `version` command, which was an example of a very simple test setup; and for the `export` command, which was more involved due to the server and client context requirements.

I did notice that there are some existing tests for some utilities, but the `cmdtest` package follows a simple pattern that has been easy to use successfully in [the relayer](https://github.com/cosmos/relayer/blob/main/internal/relayertest/system.go) and in other projects outside the Cosmos ecosystem.

While filling in these tests, I started removing uses of `cmd.Print`, as that is the root cause of issues like #8498, #7964, #15167, and possibly others. Internal to cobra, the print family of methods write to `cmd.OutOrStderr()` -- meaning that if the authors call `cmd.SetOutput()` before executing the command, the output will be written to stdout as expected; otherwise it will go to stderr. I don't understand why that would be the default behavior, but it is probably too late to change from cobra's side.

Instead of `cmd.Print`, we prefer to `fmt.Fprint(cmd.OutOrStdout())` or `fmt.Fprint(cmd.ErrOrStderr())` as appropriate, giving an unambiguous destination for output. And the new tests collect those outputs in plain `bytes.Buffer` values so that we can assert their content appropriately.

In the longer term, I would like to deprecate and eventually remove the `testutil` package's `ApplyMockIO` method and its `BufferWriter` and `BufferReader` types, as they are unnecessary indirection when a simpler solution exists. But that can wait until `cmdtest` has propagated through the codebase more.

---

### Author Checklist

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I have...

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- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#pr-targeting))
- [ ] ~~provided a link to the relevant issue or specification~~
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- [ ] confirmed all CI checks have passed

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I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed all author checklist items have been addressed
- [ ] confirmed that this PR does not change production code

server/export.go

@@ -1,8 +1,10 @@
 package server
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
 
 	"github.com/spf13/cobra"
@@ -24,7 +26,8 @@ func ExportCmd(appExporter types.AppExporter, defaultNodeHome string) *cobra.Com
 	cmd := &cobra.Command{
 		Use:   "export",
 		Short: "Export state to JSON",
-		RunE: func(cmd *cobra.Command, args []string) error {
+		Args:  cobra.NoArgs,
+		RunE: func(cmd *cobra.Command, _ []string) error {
 			serverCtx := GetServerContextFromCmd(cmd)
 			config := serverCtx.Config
 
@@ -41,16 +44,24 @@ func ExportCmd(appExporter types.AppExporter, defaultNodeHome string) *cobra.Com
 			}
 
 			if appExporter == nil {
-				if _, err := fmt.Fprintln(os.Stderr, "WARNING: App exporter not defined. Returning genesis file."); err != nil {
+				if _, err := fmt.Fprintln(cmd.ErrOrStderr(), "WARNING: App exporter not defined. Returning genesis file."); err != nil {
 					return err
 				}
 
-				genesis, err := os.ReadFile(config.GenesisFile())
+				// Open file in read-only mode so we can copy it to stdout.
+				// It is possible that the genesis file is large,
+				// so we don't need to read it all into memory
+				// before we stream it out.
+				f, err := os.OpenFile(config.GenesisFile(), os.O_RDONLY, 0)
 				if err != nil {
 					return err
 				}
+				defer f.Close()
+
+				if _, err := io.Copy(cmd.OutOrStdout(), f); err != nil {
+					return err
+				}
 
-				fmt.Println(string(genesis))
 				return nil
 			}
 
@@ -68,7 +79,7 @@ func ExportCmd(appExporter types.AppExporter, defaultNodeHome string) *cobra.Com
 
 			exported, err := appExporter(serverCtx.Logger, db, traceWriter, height, forZeroHeight, jailAllowedAddrs, serverCtx.Viper, modulesToExport)
 			if err != nil {
-				return fmt.Errorf("error exporting state: %v", err)
+				return fmt.Errorf("error exporting state: %w", err)
 			}
 
 			appGenesis, err := genutiltypes.AppGenesisFromFile(serverCtx.Config.GenesisFile())
@@ -85,12 +96,10 @@ func ExportCmd(appExporter types.AppExporter, defaultNodeHome string) *cobra.Com
 				return err
 			}
 
-			cmd.SetOut(cmd.OutOrStdout())
-			cmd.SetErr(cmd.OutOrStderr())
-
 			if outputDocument == "" {
-				cmd.Println(string(out))
-				return nil
+				// Copy the entire genesis file to stdout.
+				_, err := io.Copy(cmd.OutOrStdout(), bytes.NewReader(out))
+				return err
 			}
 
 			if err = appGenesis.SaveAs(outputDocument); err != nil {