Revert "Make lots of tests run in parallel"

This reverts commit 323dc60712.

Reason for revert: Possible cause of test instability
Bug: 170513220
Test: soong tests

Change-Id: Iee168e9fbb4210569e6cffcc23e60d111403abb8

android/prebuilt_test.go

@@ -262,7 +262,6 @@ var prebuiltsTests = []struct {
 }
 
 func TestPrebuilts(t *testing.T) {
-	t.Parallel()
 	fs := map[string][]byte{
 		"prebuilt_file": nil,
 		"source_file":   nil,