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

cmd/extract_apks/main_test.go

@@ -37,6 +37,7 @@ type testDesc struct {
 }
 
 func TestSelectApks_ApkSet(t *testing.T) {
+	t.Parallel()
 	testCases := []testDesc{
 		{
 			protoText: `
@@ -266,6 +267,7 @@ variant {
 }
 
 func TestSelectApks_ApexSet(t *testing.T) {
+	t.Parallel()
 	testCases := []testDesc{
 		{
 			protoText: `
@@ -442,6 +444,7 @@ type testCaseWriteApks struct {
 }
 
 func TestWriteApks(t *testing.T) {
+	t.Parallel()
 	testCases := []testCaseWriteApks{
 		{
 			name:       "splits",