It happened couple of times that running our selftests produced a new
entries in the default results directory. Let's use this optional check
to avoid such issue.

To enable this check one has to set "AVOCADO_RESULTSDIR_CHECK" env
variable. This check is enabled in travis as I do not expect travis to
run avocado tests while running the selfcheck, unlike on developer
computers.

Note the check is rather simple and only assumes the default results
directory. For selfcheck I think it's enough.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>