Fix NOTIFY wakeups for pre-commit LISTEN entries.

Fix NOTIFY wakeups for pre-commit LISTEN entries.

Commit 282b1cde9 made SignalBackends() ignore ListenerEntry entries
whose "listening" flag said that the listener was not yet committed.
That will be true for a new listener that has already registered its
queue position, but has not yet reached AtCommit_Notify().  If another
backend notifies the same channel in that window, SignalBackends()
would directly advance the new listener's queue position, causing it
to miss message(s).  Really this is a definitional question: is a new
listener active as of PreCommit, or as of AtCommit?  But it seems to
make more sense to expect that the new listener will see all messages
after its initially-registered queue position, especially since the
direct-advance logic is supposed to be an optimization that doesn't
affect semantics.

Fix this by treating all channel entries as valid wakeup targets.
Rename the "listening" flag to removeOnAbort to reflect its remaining
purpose: identifying staged LISTEN entries that abort cleanup must
remove.

While we're here, remove an obsolete test case added by 282b1cde9.
The check for "ChannelHashAddListener array growth" was meant to
exercise code that never made it into the committed patch, so now
it's just a waste of test cycles.

Author: Joel Jacobson <joel@compiler.org>
Reviewed-by: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/9835b0a4-9121-47ac-9c44-427b8b1a7f1b@app.fastmail.com
Discussion: https://postgr.es/m/6fe5ee75-537d-4d4f-909a-b21303c3ce75@app.fastmail.com