test: avoid redundant standby catchup in 049_wait_for_lsn

Hi Alexander, Hackers,

While working on adding more edge-case tests and fixing the timeline
handling for WAIT FOR LSN, I noticed that the overall runtime of the
test had increased by about 7 seconds since a8b61c23c5ff. I looked
into the slowdown and found a potential source.

Currently, the test creates the function, waits for the standby to
catch up, tests it, then creates the procedure and waits for the
standby to catch up again. Since both objects are only used by the
same block of top-level statement checks, we can create them together
in a single primary-side transaction and perform just one
wait_for_catchup() before running both standby-side calls.

This small TAP cleanup merges the creation of the PL/pgSQL wrapper
function and procedure used for the top-level WAIT FOR checks in
049_wait_for_lsn.pl.

The change preserves the same coverage while removing one redundant
replay catch-up on the delayed standby. It appears to reduce the test
runtime by about 7 seconds, though I have looked into why much of the
improvement comes from this change alone.

Patch attached.

Thanks.
--
Best,
Xuneng