Make lots of tests run in parallel

Putting t.Parallel() in each test makes them run in parallel.
Additional t.Parallel() could be added to each subtest, although
that requires making a local copy of the loop variable for
table driven tests.

Test: m checkbuild
Change-Id: I5d9869ead441093f4d7c5757f2447385333a95a4
diff --git a/cc/proto_test.go b/cc/proto_test.go
index f8bbd26..2d059eb 100644
--- a/cc/proto_test.go
+++ b/cc/proto_test.go
@@ -22,6 +22,7 @@
 )
 
 func TestProto(t *testing.T) {
+	t.Parallel()
 	t.Run("simple", func(t *testing.T) {
 		ctx := testCc(t, `
 		cc_library_shared {