cgo: don't run tests in parallel

Since LLVM 9, CGo sometimes randomly breaks with weird error messages on
Windows. I'm not sure why this is the case, but it might be related to
concurrency.

Disable concurrency for now, and hope that will make the errors go away.

cgo/cgo_test.go

@@ -25,8 +25,6 @@ func TestCGo(t *testing.T) {
 	for _, name := range []string{"basic", "errors", "types", "flags"} {
 		name := name // avoid a race condition
 		t.Run(name, func(t *testing.T) {
-			t.Parallel()
-
 			// Read the AST in memory.
 			path := filepath.Join("testdata", name+".go")
 			fset := token.NewFileSet()