Make more cc tests parallel

Test: go test soong cc tests
Change-Id: I5ade33b1579ae46571728e051471147e627a7d38
diff --git a/cc/library_headers_test.go b/cc/library_headers_test.go
index 3e448ba..1924b2f 100644
--- a/cc/library_headers_test.go
+++ b/cc/library_headers_test.go
@@ -59,6 +59,7 @@
 }
 
 func TestPrebuiltLibraryHeadersPreferred(t *testing.T) {
+	t.Parallel()
 	bp := `
 		cc_library_headers {
 			name: "headers",