run-tests: do not prompt changes (-i) if a race condition is detected The race condition is like: 1. run-tests.py reads test-a.t as reference output, content A 2. run-tests.py runs the test (which could be content B, another race condition fixed by the next patch, but assume it's content A here) 3. something changes test-a.t to content C 4. run-tests.py compares test output (content D) with content A 5. with "-i", run-tests.py prompts diff(A, D), while the file has content C instead of A at this time This patch detects the above case and tell the user to rerun the test if they want to apply test changes.