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/ui/build/cleanbuild_test.go b/ui/build/cleanbuild_test.go
index 89f4ad9..be60362 100644
--- a/ui/build/cleanbuild_test.go
+++ b/ui/build/cleanbuild_test.go
@@ -27,6 +27,7 @@
 )
 
 func TestCleanOldFiles(t *testing.T) {
+	t.Parallel()
 	dir, err := ioutil.TempDir("", "testcleanoldfiles")
 	if err != nil {
 		t.Fatal(err)