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

xml/xml_test.go

@@ -78,7 +78,6 @@ func assertEqual(t *testing.T, name, expected, actual string) {
 
 // Minimal test
 func TestPrebuiltEtcXml(t *testing.T) {
-	t.Parallel()
 	ctx := testXml(t, `
 		prebuilt_etc_xml {
 			name: "foo.xml",