test(auth): enable skipped quota project default credentials testcase (#13459)

In DefaultCredentialsProviderTest.java, the
getDefaultCredentials_quota_project test case was missing the JUnit
@test annotation, causing it to be ignored. This adds the annotation to
ensure it is executed during test runs.

Author: westarle

google-auth-library-java/oauth2_http/javatests/com/google/auth/oauth2/DefaultCredentialsProviderTest.java

@@ -387,6 +387,7 @@ void getDefaultCredentials_GdchServiceAccount() throws IOException {
     assertNotNull(((GdchCredentials) defaultCredentials).getApiAudience());
   }
 
+  @Test
   void getDefaultCredentials_quota_project() throws IOException {
     InputStream userStream =
         UserCredentialsTest.writeUserStream(