Show GHA warnings for slow tests when GHA is detected

[mvorisek]

inspired by PHP-CS-Fixer/PHP-CS-Fixer#9025 (comment)

Example of GHA warning on PR:

Currently, the output can be overseen on PRs, this is a feature request to format the slow tests as GHA warnings for corresponding methods when GHA is detected.

[Wirone]

Annotating test methods (or data providers' cases) is nice to have, but outputting basic warning that would be displayed in the workflow's summary is good enough to start with.

[mvorisek]

Inspiration can be taken from PHP-CS-Fixer/PHP-CS-Fixer#9025 (comment).