use apex_test for test apexes

... so that the build system can distinguish testing APEXes and installable APEXes, and apply
different rules to them.

Bug: 146308764
Test: m
Change-Id: Ia78a334c136bf8aaee448dbd2f2f99f593440d93
diff --git a/apex/testing/Android.bp b/apex/testing/Android.bp
index 549b6e3..5a72164 100644
--- a/apex/testing/Android.bp
+++ b/apex/testing/Android.bp
@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-apex {
+apex_test {
     name: "test_com.android.cronet",
     visibility: [
         "//system/apex/tests",