Allow more time for credentials tests

The switch from a single JenkinsRule instance at the class level to a
JenkinsRule per credentials test slows the individual tests but should
allow more total tests to be executed because the 180 second JenkinsRule
timeout won't be reached inside any single JenkinsRule test.

Still want to limit the total execution time of the CredentialsTest
class to not exceed 3 minutes.  Otherwise, total test execution time
becomes too long.

src/test/java/org/jenkinsci/plugins/gitclient/CredentialsTest.java

@@ -350,7 +350,7 @@ private void addCredential() throws IOException {
      * @return true if another test should be allowed to start
      */
     private boolean testPeriodNotExpired() {
-        return (System.currentTimeMillis() - firstTestStartTime) < ((180 - 120) * 1000L);
+        return (System.currentTimeMillis() - firstTestStartTime) < ((180 - 30) * 1000L);
     }
 
     @Test