tests/helper-runtests.sh

merge: move the filtering of ambiguous files to a dedicated function

I have multiple reasons:
	- The body of `_update` is way too long
	- This adds typing which will help our tooling and brains understand this
	  code more easily
	- This function will get more nested and complex in the next patch

I've taken the liberty of rewrapping and typo-passing the docstring.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL