cmd/compile/internal/syntax: don't try to parse files in GOROOT/.git

This test was failing locally in my clone of the go repo due to a Git
branch ending in ".go", which the test found and was attempting to
parse as a file. It's fragile to try to parse .go files in
GOROOT/.git, and wasteful to scan GOROOT/pkg and other non-source
directories; instead, let's only parse the directories we actually
expect to contain source files.

(I was running the test for #51461.)

Change-Id: I5d4e31ec2bcd9b4b6840ec32ad9b12bf44f349a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/390023
Trust: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: Bryan C. Mills

src/cmd/compile/internal/syntax/parser_test.go

@@ -78,7 +78,8 @@ func TestStdLib(t *testing.T) {
 	go func() {
 		defer close(results)
 		for _, dir := range []string{
-			runtime.GOROOT(),
+			filepath.Join(runtime.GOROOT(), "src"),
+			filepath.Join(runtime.GOROOT(), "misc"),
 		} {
 			walkDirs(t, dir, func(filename string) {
 				if skipRx != nil && skipRx.MatchString(filename) {