go/packages: fix data race in TestCgoNoSyntax

CL 332350 introduced a race to exported.Config.Mode when it parallelized
this test. Revert the call to t.Parallel.

Change-Id: Ifd4b6c4037ee30e8b3f55dc69d4f9f3f09a89185
Reviewed-on: https://go-review.googlesource.com/c/tools/+/333509
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/go/packages/packages_test.go b/go/packages/packages_test.go
index afc0a80..61cd384 100644
--- a/go/packages/packages_test.go
+++ b/go/packages/packages_test.go
@@ -1905,8 +1905,6 @@
 	for _, mode := range modes {
 		mode := mode
 		t.Run(fmt.Sprint(mode), func(t *testing.T) {
-			t.Parallel()
-
 			exported.Config.Mode = mode
 			pkgs, err := packages.Load(exported.Config, "golang.org/fake/c")
 			if err != nil {