Debug output when watcher decides to run only exclusive tests

[novemberborn]

Users can use the DEBUG=ava:watcher environment variable to get insight into the decisions made by the watcher. This needs to be extended for the sticky .only behavior.

With sticky .only, if the watcher detects a .only in one file, and then needs to run tests from another, it forces the Api to run only the exclusive tests from that second file. The watcher should output which files containing exclusive tests were not rerun that forced the Api to run only exclusive tests.

[novemberborn]

Since #733 all files containing exclusive tests are rerun. Debug output is no longer necessary.