ConfigCacheClientTest.java: fix flaky test due to race condition

firebase-config/src/test/java/com/google/firebase/remoteconfig/internal/ConfigCacheClientTest.java

@@ -42,6 +42,7 @@
 import org.mockito.ArgumentCaptor;
 import org.mockito.Mock;
 import org.mockito.MockitoAnnotations;
+import org.mockito.stubbing.Answer;
 import org.robolectric.RobolectricTestRunner;
 import org.robolectric.annotation.Config;
 
@@ -220,7 +221,7 @@ public void getBlocking_hasCachedValue_returnsCache() throws Exception {
 
   @Test
   public void getBlocking_hasNoCachedValueAndFileReadTimesOut_returnsNull() throws Exception {
-    when(mockStorageClient.read()).thenReturn(configContainer);
+    when(mockStorageClient.read()).thenAnswer(BLOCK_INDEFINITELY);
 
     ConfigContainer container = cacheClient.getBlocking(/* diskReadTimeoutInSeconds= */ 0L);
 
@@ -329,4 +330,18 @@ public void cleanUp() {
     cacheThreadPool.shutdownNow();
     testingThreadPool.shutdownNow();
   }
+
+  /**
+   * A Mockito "answer" that blocks indefinitely. The only way that {@link Answer#answer} will
+   * return is if its thread is interrupted. This may be useful to cause a method to never return,
+   * which should result in a timeout waiting for the operation to complete.
+   * <p>
+   * Example:
+   * {@code when(foo.get()).thenAnswer(BLOCK_INDEFINITELY); }
+   */
+  private static final Answer<ConfigContainer> BLOCK_INDEFINITELY =
+      invocation -> {
+        Thread.sleep(Long.MAX_VALUE);
+        throw new RuntimeException("BLOCK_INDEFINITELY.answer() should never get here");
+      };
 }