Create RDS test clients with fixture

[snordhausen]

Create RDS test clients with a fixture to get rid of copy&paste code in the tests. The variable is now consistently called client.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.57%. Comparing base (479c211) to head (7b25083).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8406      +/-   ##
==========================================
- Coverage   94.59%   94.57%   -0.03%     
==========================================
  Files        1163     1163              
  Lines      101636   101636              
==========================================
- Hits        96147    96119      -28     
- Misses       5489     5517      +28     

Flag	Coverage Δ
servertests	?
unittests	94.57% <ø> (ø)

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[snordhausen]

I'll look into what's breaking the server tests, I can reproduce the issue locally.

[bpandola]

@snordhausen This may help: https://docs.getmoto.org/en/latest/docs/getting_started.html#pytest-fixtures-example-usage