feat(std): support timestamp skipping in test cases (#569)

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# Description

support timestamp skipping in test cases

# How has this been tested?

test case run by gnodev

docs/reference/stdlibs/std/testing.md

@@ -20,6 +20,8 @@ func TestSkipHeights(count int64)
 ```
 Modifies the block height variable by skipping **count** blocks.
 
+It also increases block timestamp by 5 seconds for every single count
+
 #### Usage
 ```go
 std.TestSkipHeights(100)

examples/gno.land/p/demo/rand/rand0_filetest.gno

@@ -49,8 +49,8 @@ func main() {
 // 177
 // 802
 // ---
-// 450
-// 78
-// 777
-// 15
-// 339
+// 269
+// 233
+// 591
+// 936
+// 908

examples/gno.land/r/x/skip_height_to_skip_time/skiptime.gno

@@ -0,0 +1 @@
+package skiptime

examples/gno.land/r/x/skip_height_to_skip_time/skiptime_test.gno

@@ -0,0 +1,27 @@
+package skiptime
+
+import (
+	"std"
+	"testing"
+	"time"
+)
+
+func TestSkipHeights(t *testing.T) {
+	oldHeight := std.GetHeight()
+	shouldEQ(t, oldHeight, 123)
+
+	oldNow := time.Now().Unix()
+	shouldEQ(t, oldNow, 1234567890)
+
+	// skip 3 blocks == 15 seconds
+	std.TestSkipHeights(3)
+
+	shouldEQ(t, std.GetHeight()-oldHeight, 3)
+	shouldEQ(t, time.Now().Unix()-oldNow, 15)
+}
+
+func shouldEQ(t *testing.T, got, expected int64) {
+	if got != expected {
+		t.Fatalf("expected %d, got %d.", expected, got)
+	}
+}

gnovm/tests/stdlibs/std/std.go

@@ -57,6 +57,7 @@ func IsOriginCall(m *gno.Machine) bool {
 func TestSkipHeights(m *gno.Machine, count int64) {
 	ctx := m.Context.(*TestExecContext)
 	ctx.Height += count
+	ctx.Timestamp += (count * 5)
 	m.Context = ctx
 }