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/android/androidmk_test.go b/android/androidmk_test.go
index a558f45..8574dc9 100644
--- a/android/androidmk_test.go
+++ b/android/androidmk_test.go
@@ -68,6 +68,7 @@
 }
 
 func TestAndroidMkSingleton_PassesUpdatedAndroidMkDataToCustomCallback(t *testing.T) {
+	t.Parallel()
 	bp := `
 	custom {
 		name: "foo",
@@ -103,6 +104,7 @@
 }
 
 func TestGetDistForGoals(t *testing.T) {
+	t.Parallel()
 	testCases := []struct {
 		bp                     string
 		expectedAndroidMkLines []string