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/library_headers_test.go b/cc/library_headers_test.go
index 564ef61..db42d25 100644
--- a/cc/library_headers_test.go
+++ b/cc/library_headers_test.go
@@ -20,6 +20,7 @@
 )
 
 func TestLibraryHeaders(t *testing.T) {
+	t.Parallel()
 	ctx := testCc(t, `
 	cc_library_headers {
 		name: "headers",
@@ -41,6 +42,7 @@
 }
 
 func TestPrebuiltLibraryHeaders(t *testing.T) {
+	t.Parallel()
 	ctx := testCc(t, `
 	cc_prebuilt_library_headers {
 		name: "headers",