tests: apply KISS to retry fixture

tests: apply KISS to retry fixture

This python fixture was way too complex for what is needed.

Eliminate gratuitous options/over-engineering:

- Change from non-deterministic `wait` and `attempts` to a single
`retry_timeout` value. This is both more deterministic, as well as
what the user should actually be thinking about.

- Use a fixed 2 second pause between executing the wrapped function
rather than a bunch of arbitrary choices of 2, 3 and 4 seconds
spread all over the test code.

- Get rid of the multiple variables for determining what "Positive" and
"Negative" results are. Instead just implement what all the user code
already wants, i.e., boolean False or a str (errormsg) means
"Negative" result otherwise it's a "Positive" result.

- As part of the above the inversion logic is much more comprehensible
in the fixture code (and more correct to boot).

Signed-off-by: Christian Hopps <chopps@labn.net>