chore: test TEST server first before some tests around getProductV3

[monsieurtanuki]

What

• The purpose of this PR is to get rid of annoying emails about some specific tests non passing.
• In that particular case, the TEST server is often funny in its head, and getProductV3 (and of course the corresponding URL) systematically fails with a 502 Bad Gateway error.
• Instead of receiving error messages about problems that do not regard our code but the current status of the TEST env, we check first if a simple getProductV3 call works. If it triggers a 502, we ignore the test. If it passes, we run the test. IF there's another exception, we fail the test.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (3350251) 75.88% compared to head (bddbd69) 73.00%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
test/api_get_save_product_test.dart	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #859      +/-   ##
==========================================
- Coverage   75.88%   73.00%   -2.89%     
==========================================
  Files         224      224              
  Lines        7880     7895      +15     
==========================================
- Hits         5980     5764     -216     
- Misses       1900     2131     +231     

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[M123-dev]

It would be amazing to stop these (for me push notifications from the Github App), so 100% for this check 💯

[monsieurtanuki]

Thank you @M123-dev for your review!

Let's extend the same principle each time we have "irrelevant" test errors. For that we'll need a simple query on the related server.
We can do that "on demand", next time the next tests fail for no "good" reason.