gio/tests/g-file-info: don't assume million-in-one events don't happen

The access and creation time tests create a file, gets the time in seconds, then gets the time in microseconds and assumes that the difference between the two has to be above 0.

As rare as this may be, it can happen:

$ stat g-file-info-test-50A450 -c %y 2021-07-06 18:24:56.000000767 +0100

Change the test to simply assert that the difference not negative to handle this case.

This is the same fix as 289f8be1, but that was just modification time.