Improve runtime and output of tests for replication slots checkpointing.

Improve runtime and output of tests for replication slots checkpointing.

The TAP tests that verify logical and physical replication slot behavior
during checkpoints (046_checkpoint_logical_slot.pl and
047_checkpoint_physical_slot.pl) inserted two batches of 2 million rows each,
generating approximately 520 MB of WAL.  On slow machines, or when compiled
with '-DRELCACHE_FORCE_RELEASE -DCATCACHE_FORCE_RELEASE', this caused the
tests to run for 8-9 minutes and occasionally time out, as seen on the
buildfarm animal prion.

This commit modifies the mentioned tests to utilize the $node->advance_wal()
function, thereby reducing runtime. Once we do not use the generated data,
the proposed function is a good alternative, which cuts the total wall-clock
run time.

While here, remove superfluous '\n' characters from several note() calls;
these appeared literally in the build-farm logs and looked odd.  Also, remove
excessive 'shared_preload_libraries' GUC from the config and add a check for
'injection_points' extension availability.

Reported-by: Alexander Lakhin <[email protected]>
Reported-by: Tom Lane <[email protected]>
Author: Alexander Korotkov <[email protected]>
Author: Vitaly Davydov <[email protected]>
Reviewed-by: Hayato Kuroda <[email protected]>
Discussion: https://postgr.es/m/fbc5d94e-6fbd-4a64-85d4-c9e284a58eb2%40gmail.com
Backpatch-through: 17