ci: Add swagger ping in API CI job

[frgfm]

Following up on #847, this PR introduces the following modification:

• adds a step in CI job for API, to ping the swagger before running the unittest

Presumably, if the behaviour described in #847, the CI should fail on this PR.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #906 (de829ce) into main (3d645ce) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #906   +/-   ##
=======================================
  Coverage   94.73%   94.73%           
=======================================
  Files         134      134           
  Lines        5472     5472           
=======================================
  Hits         5184     5184           
  Misses        288      288           

Flag	Coverage Δ
unittests	94.73% <ø> (ø)

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[charlesmindee]

Thanks for this!