[MRG] split gromov test file

[cedricvincentcuaz]

Types of changes

split too long test file test/test_gromov.py within a subfolder test/gromov/.

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

Merging #619 (f93dcba) into master (ab12dd6) will increase coverage by 0.00%.
The diff coverage is 99.39%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #619   +/-   ##
=======================================
  Coverage   96.79%   96.79%           
=======================================
  Files          77       81    +4     
  Lines       16086    16098   +12     
=======================================
+ Hits        15570    15582   +12     
  Misses        516      516