The t-20-bad_configs test can sometimes have false positives because
the last line recorded in the log isn't always the one causing the
failure, due to asynchronous logging and in particular in combination
with coverage tests (which alter the os.Exit behaviour subtly).
This patch fixes that by having the tests look at the last 4 lines of
output instead. This is not super pretty, but it should be good enough
to cover for any timing issues that arise in these particular tests.
