go/internal/gcimporter: skip TestImportTypeparamTests if GOROOT/test is missing

cmd/distpack removes GOROOT/test, so skip the test if it isn't there.

For #24904.

Change-Id: Iac381517d0540056b2ccea0dc1bd716113b18468
Reviewed-on: https://go-review.googlesource.com/c/go/+/504117
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/go/internal/gcimporter/gcimporter_test.go

@@ -137,6 +137,16 @@ func TestImportTypeparamTests(t *testing.T) {
 		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
 	}
 
+	// cmd/distpack removes the GOROOT/test directory, so skip if it isn't there.
+	// cmd/distpack also requires the presence of GOROOT/VERSION, so use that to
+	// avoid false-positive skips.
+	gorootTest := filepath.Join(testenv.GOROOT(t), "test")
+	if _, err := os.Stat(gorootTest); os.IsNotExist(err) {
+		if _, err := os.Stat(filepath.Join(testenv.GOROOT(t), "VERSION")); err == nil {
+			t.Skipf("skipping: GOROOT/test not present")
+		}
+	}
+
 	testenv.MustHaveGoBuild(t)
 
 	tmpdir := mktmpdir(t)
@@ -144,7 +154,7 @@ func TestImportTypeparamTests(t *testing.T) {
 
 	// Check go files in test/typeparam, except those that fail for a known
 	// reason.
-	rootDir := filepath.Join(testenv.GOROOT(t), "test", "typeparam")
+	rootDir := filepath.Join(gorootTest, "typeparam")
 	list, err := os.ReadDir(rootDir)
 	if err != nil {
 		t.Fatal(err)