Make apex tests parallel

Mark all the apex tests and subtests with t.Parallel(), which will allow
the test runner to run them in parallel.  Reduces the time required to
run the apex tests from 70s to 9.7s.

Test: go test ./apex
Flag: EXEMPT test only
Change-Id: I08bd3cb4d8eec1f9520c1118b4525aa79e95288e
diff --git a/apex/dexpreopt_bootjars_test.go b/apex/dexpreopt_bootjars_test.go
index 4feade8..b51bb36 100644
--- a/apex/dexpreopt_bootjars_test.go
+++ b/apex/dexpreopt_bootjars_test.go
@@ -168,6 +168,7 @@
 }
 
 func TestDexpreoptBootJarsWithSourceArtApex(t *testing.T) {
+	t.Parallel()
 	ruleFile := "out/soong/dexpreopt_arm64/dex_bootjars/android/system/framework/arm64/boot.art"
 
 	expectedInputs := []string{
@@ -206,6 +207,7 @@
 // The only difference is that the ART profile should be deapexed from the prebuilt APEX. Other
 // inputs and outputs should be the same as above.
 func TestDexpreoptBootJarsWithPrebuiltArtApex(t *testing.T) {
+	t.Parallel()
 	ruleFile := "out/soong/dexpreopt_arm64/dex_bootjars/android/system/framework/arm64/boot.art"
 
 	expectedInputs := []string{
@@ -243,6 +245,7 @@
 
 // Changes to the boot.zip structure may break the ART APK scanner.
 func TestDexpreoptBootZip(t *testing.T) {
+	t.Parallel()
 	ruleFile := "boot.zip"
 
 	ctx := android.PathContextForTesting(android.TestArchConfig("", nil, "", nil))
@@ -271,6 +274,7 @@
 // Multiple ART apexes might exist in the tree.
 // The profile should correspond to the apex selected using release build flags
 func TestDexpreoptProfileWithMultiplePrebuiltArtApexes(t *testing.T) {
+	t.Parallel()
 	ruleFile := "out/soong/dexpreopt_arm64/dex_bootjars/android/system/framework/arm64/boot.art"
 	bp := `
 		// Platform.
@@ -425,6 +429,7 @@
 
 // Check that dexpreopt works with Google mainline prebuilts even in workspaces where source is missing
 func TestDexpreoptWithMainlinePrebuiltNoSource(t *testing.T) {
+	t.Parallel()
 	bp := `
 		// Platform.