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/apex_test.go b/android/apex_test.go
index 512b50f..7507628 100644
--- a/android/apex_test.go
+++ b/android/apex_test.go
@@ -20,6 +20,7 @@
 )
 
 func Test_mergeApexVariations(t *testing.T) {
+	t.Parallel()
 	tests := []struct {
 		name        string
 		in          []ApexInfo