gh-155648: In IDLE tests, call unittest.main without exit arg

[terryjreedy]

DD bug 73: In idlelib.idle_test, test_xyz.py files should end with

if __name__ == '__main__':
    unittest.main(verbosity=2)

The default exit is True. This need not and should not be added.
5 files add the confusing equivalent exit=2 ("why the weird value?"),

4 files add exit=False. This is nonsensical when there is nothing more to run;
main will immediately exit anyway. When running a test file from an IDLE editor,
this argument has no visible effect. However, a Claude-based
bug finder claims that in other circumstances (such as a program running
the test in a shell), the good test may falsely fail. Even if this is
not true, it can only confuse a reader.

(The only place in idlelib for exit=False is in idlelib/abc.py files where the unittest
is followed by an htest. The default exit=True exits the process, skipping the htest.)

As part of editing the discussion of this in idle_test/htest.py, I clarified other things.

• Issue: IDLE - fix devdanzin test issues #155648

[miss-islington-app]

Thanks @terryjreedy for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-156251 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-156252 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-156253 is a backport of this pull request to the 3.13 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Fedora Stable Clang 3.x (tier-2) has failed when building commit c20318b.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/441/builds/10297) and take a look at the build logs.
4. Check if the failure is related to this commit (c20318b) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/441/builds/10297

Summary of the results of the build (if available):

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Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at c20318bf09e59 gh-155648: In IDLE tests, call unittest.main without exit arg (#156249)
Switched to and reset branch 'main'

ar: unable to copy file 'libpython3.16d.a'; reason: No space left on device
make: *** [Makefile:1164: libpython3.16d.a] Error 1