Check which files were rechecked in incremental mode test cases (and which weren't)

[gvanrossum]

We need to improve the test harness for incremental tests (in testcheck.py) to check which files were or weren't rechecked. We probably need to add new syntax to test descriptions to describe expectations here.

[Michael0x2a]

As an update: the new syntax was added in #1811.

[ilevkivskyi]

This is now supported by [rechecked], [stale], etc.