test: fix flaky concurrent test in cab token generator

The refreshCredentialsIfRequired_asyncMultiThread test was failing because the mock clock was returning a fixed stale time, causing multiple threads to trigger redundant refreshes if they didn't overlap perfectly.

This change modifies the mock clock to return a stale time on the first call (to trigger the async refresh) and a fresh time on subsequent calls (to skip redundant refreshes). This fix is isolated to only the ASYNC test case to avoid breaking BLOCKING tests.

Author: meltsufin

google-auth-library-java/cab-token-generator/javatests/com/google/auth/credentialaccessboundary/ClientSideCredentialAccessBoundaryFactoryTest.java

@@ -614,22 +614,30 @@ private Clock createMockClock(RefreshType refreshType, GoogleCredentials sourceC
         // Set mocked time so that the token is fresh and no refresh is needed (before the refresh
         // margin).
         mockedTimeInMillis = expirationTimeInMillis - refreshMarginInMillis - 60000;
+        when(mockClock.currentTimeMillis()).thenReturn(mockedTimeInMillis);
         break;
       case ASYNC:
         // Set mocked time so that the token is nearing expiry and an async refresh is triggered
         // (within the refresh margin).
         mockedTimeInMillis = expirationTimeInMillis - refreshMarginInMillis + 60000;
+        when(mockClock.currentTimeMillis())
+            .thenReturn(
+                mockedTimeInMillis, // First call: Stale (triggers the async refresh)
+                currentTimeInMillis // Subsequent calls: Fresh (skips redundant refreshes)
+            );
         break;
       case BLOCKING:
         // Set mocked time so that the token requires immediate refresh (just after the minimum
         // token lifetime).
         mockedTimeInMillis = expirationTimeInMillis - minimumTokenLifetimeMillis + 60000;
+        when(mockClock.currentTimeMillis()).thenReturn(mockedTimeInMillis);
         break;
       default:
         throw new IllegalArgumentException("Unexpected RefreshType: " + refreshType);
     }
 
-    when(mockClock.currentTimeMillis()).thenReturn(mockedTimeInMillis);
+
+
     return mockClock;
   }