runtests.py's bisect_tests() and paired_tests() don't always need to do setup and teardown

[Chris Jerdonek]

Currently, runtests.py's ​bisect_tests() and ​paired_tests() both call runtests.py's setup() and teardown(). However, it doesn't seem like they need to (in full).

The setup() function's primary purpose is to temporarily set settings for the purposes of the test run, but bisect_tests() and paired_tests() both run tests in a subprocess, so settings modification doesn't seem like it should be necessary. The only things the call to setup() seem to be needed for in these two cases is to (1) do some initial logging of the test run, and (2) do some work to compute test_labels in the case that test_labels aren't provided. So those pieces could be factored out.

Here is the commit where the calls to setup() were first added: ​https://github.com/django/django/commit/18c3ea5546566e8fe1471ea87942b2864040452d

This is related to (but separate from) #32609.

[Chris Jerdonek]

PR: ​https://github.com/django/django/pull/14230