Add t.Parallel() to java tests

Speeds up go test ./java from 32 seconds to 9 seconds.

Test: go test ./java
Change-Id: I4c6640e787c0744c6dfda03b7382839b452e24cf
diff --git a/java/prebuilt_apis_test.go b/java/prebuilt_apis_test.go
index b6fb2c6..c6a5913 100644
--- a/java/prebuilt_apis_test.go
+++ b/java/prebuilt_apis_test.go
@@ -29,6 +29,7 @@
 }
 
 func TestPrebuiltApis_SystemModulesCreation(t *testing.T) {
+	t.Parallel()
 	result := android.GroupFixturePreparers(
 		prepareForJavaTest,
 		FixtureWithPrebuiltApis(map[string][]string{
@@ -61,6 +62,7 @@
 }
 
 func TestPrebuiltApis_WithExtensions(t *testing.T) {
+	t.Parallel()
 	runTestWithBaseExtensionLevel := func(v int) (foo_input, bar_input, baz_input string) {
 		result := android.GroupFixturePreparers(
 			prepareForJavaTest,
@@ -101,6 +103,7 @@
 }
 
 func TestPrebuiltApis_WithIncrementalApi(t *testing.T) {
+	t.Parallel()
 	runTestWithIncrementalApi := func() (foo_input, bar_input, baz_input string) {
 		result := android.GroupFixturePreparers(
 			prepareForJavaTest,