gh-155648: In IDLE tests, call unittest.main without exit arg (#156249)

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

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.