pytest tests: Enhanced assertion for check_* methods.

Checking if some output is present or absent from standard streams was
done by simple string searching. Due to recent changes in FreeIPA, this
search is not effective due to capitalization differences in boolean
values output. Changing the string searching to regular expression
searches fixes this behavior for current and previous versions of
FreeIPA.

This patch also adds more information on the assert tests in case of an
error, so that it is easier to understand why the test failed.