Functional tests for SimpleCredsAuth plugin

[celestian]

This parametrized test try all possibilities of SimpleCredsAuth plugin configuration and try access Custodia server.

[codecov-io]

Codecov Report

Merging #229 into master will increase coverage by 0.28%. The diff coverage is 83.2%.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   72.74%   73.03%   +0.28%     
==========================================
  Files          41       42       +1     
  Lines        4433     4535     +102     
  Branches      449      472      +23     
==========================================
+ Hits         3225     3312      +87     
- Misses       1052     1056       +4     
- Partials      156      167      +11

Impacted Files	Coverage Δ
tests/functional/test_plugin_auth.py	100% <100%> (ø)
tests/functional/base.py	84.91% <81.66%> (-1.45%)	:arrow_down:

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[celestian]

In my opinion it could be useful to improve documentation of this plugin a bit. If we have configuration as:

[auth:creds]
handler = SimpleCredsAuth
uid = <correct_id>
gid = <incorrect_id>

the access is granted (proved by test). It could be little confusing that uid and gid are in OR relation.