Fix heisentest by ignoring the basepath

[megaserg]

Problem:
The issue #10988 suspects that there's a dependency on side-effects from other tests.
However, the test itself seems pretty hermetic, creating a new randomly-named temp directory every time.
My guess is that there is a case where the randomly-generated name contains one of the ignored tokens.
For example, probability of a random 8-character name containing "not" is roughly 6 / 37^3 ~ 0.01%.
In this case, all files will be wrongfully ignored because we use the absolute path to perform the pattern matching.

Solution:
Use the relative path to perform the pattern matching. Also, look for the first match only (no need for re.findall()).

Validation:
The modified test intentionally includes ignored token "not" in the name of the base path. With this modification, the test fails 100% of the time, unless the suggested fix in find_path_from_directory() is applied.

Closes: #10988

[boring-cyborg]

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[megaserg]

@potiuk @j-y-matsubara does this hypothesis sound plausible to you?

[potiuk]

Great findings! Indeed that looks like a plausible explanation AND it actually fixes a real problem that actual user might not have realized! Thanks @megaserg !

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[github-actions]

The PR needs to run all tests because it modifies core of Airflow! Please rebase it to latest master or ask committer to re-run it!

[potiuk]

I disagree with you bot :)