subtests interact badly with -rewrite

[RaduBerinde]

If you try to run a subtest and pass -rewrite, the outcome is bad (all tests that did not run disappear).

We should either make it work (by not modifying the tests that did not run) or detect when a test filter is in place and disallow -rewrite. One way to detect would be to count how many of our .Run() calls actually run, and bail at the end if we didn't run anything.

We should make a similar change to the logictests.