subtests interact badly with -rewrite

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.