migrate to fishtape 3

[edouard-lopez]

@jorgebucaran wanna try it?

⚠️ still a work in progress and plenty of failing tests I stop in the middle :/

• restrict _purge_configs & _disable_colors execution to test container
• add _pure_is_inside_container
• refactor methods into _pure_user_at_host
• add v3->v4 migration script to update variables
• bump version 4.0.0
• show user@host when running in a container
• bump version 4.1.0
• quote test arguments
• remove import for version-compare in test
• correct typo
• use (status filename) instead of $current_filename
• Use (dirname (status filename)) instead of $current_dirname (needs Fish >=3.2).
• replace @mesg by @echo
• explicit call to setup and teardown

[jorgebucaran]

If you are not using setup anywhere else, you might as well remove the setup function and just leave the code.

[edouard-lopez]

I prefer to keep the function, it helps explicit what the code does

[jorgebucaran]

I'd agree if we were using the function more than once in the same file, but we are not.

[edouard-lopez]

/close #272