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

android/visibility_test.go

@@ -1140,6 +1140,7 @@ var visibilityTests = []struct {
 }
 
 func TestVisibility(t *testing.T) {
+	t.Parallel()
 	for _, test := range visibilityTests {
 		t.Run(test.name, func(t *testing.T) {
 			ctx, errs := testVisibility(buildDir, test.fs)