Avoid using timezone Asia/Manila in regression tests.

Avoid using timezone Asia/Manila in regression tests.

The freshly-released 2025a version of tzdata has a refined estimate
for the longitude of Manila, changing their value for LMT in
pre-standardized-timezone days.  This changes the output of one of
our test cases.  Since we need to be able to run with system tzdata
files that may or may not contain this update, we'd better stop
making that specific test.

I switched it to use Asia/Singapore, which has a roughly similar UTC
offset.  That LMT value hasn't changed in tzdb since 2003, so we can
hope that it's well established.

I also noticed that this set of make_timestamptz tests only exercises
zones east of Greenwich, which seems rather sad, and was not the
original intent AFAICS.  (We've already changed these tests once
to stabilize their results across tzdata updates, cf 66b737cd9;
it looks like I failed to consider the UTC-offset-sign aspect then.)
To improve that, add a test with Pacific/Honolulu.  That LMT offset
is also quite old in tzdb, so we'll cross our fingers that it doesn't
get improved.

Reported-by: Christoph Berg <cb@df7cb.de>
Discussion: http://postgr.es/m/Z46inkznCxesvDEb@msg.df7cb.de
Backpatch-through: 13