tests: add more integration tests for windows/wcow

As a follow up of #5027, we will be adding more tests in batches
of 10-15, to bring the coverage at par with Linux where applicable.

Tests completed in `frontend/dockerfile`:
- [x] `testTarExporterBasic`
- [x] `testWorkdirCreatesDir`
- [x] `testCacheReleased`
- [x] `testSymlinkedDockerfile`
- [x] `testMultiArgs`
- [x] `testContextChangeDirToFile`
- [ ] `testDefaultShellAndPath` (skipped, to revisit)
- [ ] `testExportMultiPlatform` (skipped, to revisit multi-platform
  support on Windows)
- [x] `testNoSnapshotLeak`
- [x] `testCopyFollowAllSymlinks`
- [x] `testCopySymlinks`
- [x] `testHTTPDockerfile`
- [x] `testCmdShell`
- [x] `testInvalidJSONCommands` (flaky, due to port detection for
  registry.exe server)
- [x] `testGlobalArg`
- [ ] `testDockerfileDirs`
- [x] `testDockerfileInvalidCommand`
- [x] `testCopyOverrideFiles`
- [x] `testCopyVarSubstitution`
- [ ] `testCopyWildcards` (fails on WS2022)

Signed-off-by: Anthony Nandaa <[email protected]>

.github/workflows/test-os.yml

@@ -77,7 +77,7 @@ jobs:
     needs:
       - build
     env:
-      TESTFLAGS: "-v --parallel=6 --timeout=60m"
+      TESTFLAGS: "-v --timeout=60m"
       GOTESTSUM_FORMAT: "standard-verbose"
     strategy:
       fail-fast: false