util: Increase modtime in AdvancedTlsX509TrustManagerTest

I noticed an old JDK 8u275 failed on the test because the modification
time's resolution was one second. A newer JDK 8u432 worked fine, so it's
not really a problem for me, but increasing the time difference is
cheap. I used two seconds as that's the resolution available on FAT
(which is unlikely to be TMPDIR, even on Windows).

util/src/test/java/io/grpc/util/AdvancedTlsX509TrustManagerTest.java

@@ -168,7 +168,7 @@ public void updateTrustCredentials_rotate() throws GeneralSecurityException, IOE
     fakeClock.forwardTime(1, TimeUnit.MINUTES);
     assertArrayEquals(serverCert0, trustManager.getAcceptedIssuers());
 
-    serverCert0File.setLastModified(serverCert0File.lastModified() - 10);
+    serverCert0File.setLastModified(serverCert0File.lastModified() - 2000);
 
     fakeClock.forwardTime(1, TimeUnit.MINUTES);
     assertArrayEquals(serverCert0, trustManager.getAcceptedIssuers());
@@ -181,7 +181,7 @@ public void updateTrustCredentials_rotate() throws GeneralSecurityException, IOE
     fakeClock.forwardTime(1, TimeUnit.MINUTES);
     assertArrayEquals(serverCert0, trustManager.getAcceptedIssuers());
 
-    serverCert0File.setLastModified(beforeModify + 10);
+    serverCert0File.setLastModified(beforeModify + 2000);
 
     // file modification time changed
     fakeClock.forwardTime(1, TimeUnit.MINUTES);