test: wait until session pool init has finished

[olavloite]

Wait until the session pool has been initialized before verifying the number of RPC calls on the server.

Fixes #1024

[codecov]

Codecov Report

Merging #1116 (b29fa9d) into postgresql-dialect (4384ffb) will decrease coverage by 0.10%.
Report is 7 commits behind head on postgresql-dialect.
The diff coverage is n/a.

❗ Current head b29fa9d differs from pull request most recent head 8879535. Consider uploading reports for the commit 8879535 to get more accurate results

@@                   Coverage Diff                    @@
##             postgresql-dialect    #1116      +/-   ##
========================================================
- Coverage                 90.55%   90.46%   -0.10%     
+ Complexity                 2506     2503       -3     
========================================================
  Files                       140      140              
  Lines                      8334     8334              
  Branches                   1213     1213              
========================================================
- Hits                       7547     7539       -8     
- Misses                      531      536       +5     
- Partials                    256      259       +3     

Flag	Coverage Δ
all_tests	90.46% <ø> (-0.10%)	⬇️

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see 3 files with indirect coverage changes

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