blackbox: guard against recursion from dirty check

tests: divorce blackbox test from test-dispatch.py I used test-dispatch.py to demonstrate what would happen if a log file changed from being readonly to writable, by having it replace a directory (proxy for readonly/not-writable) with a log file in between transactions of a running python process (proxy for Mercurial). This commit makes it easier for people to follow what the test is doing, by creating a real file that people can read.