pgsql: Check availability of module injection_points in TAP tests

Check availability of module injection_points in TAP tests

This fixes defects with installcheck for TAP tests that expect the
module injection_points to exist in an installation, but the contents of
src/test/modules are not installed by default with installcheck. This
would cause, for example, failures under installcheck-world for a build
with injection points enabled, when the contents of src/test/modules/
are not installed.

The availability of the module can be done with a scan of
pg_available_extension. This has been introduced in 2cdcae9da696, and
it is refactored here as a new routine in Cluster.pm.

Tests are changed in different ways depending on what they need:
- The libpq TAP test sets up a node even without injection points, so it
is enough to check that CREATE EXTENSION can be used. There is no need
for the variable enable_injection_points.
- In test_misc, 006_signal_autovacuum requires a runtime check.
- 041_checkpoint_at_promote in recovery tests and 005_timeouts in
test_misc are updated to use the routine introduced in Cluster.pm.
- test_slru's 001_multixact, injection_points's 001_stats and
modules/gin/ do not require a check as these modules disable
installcheck entirely.

Discussion: https://postgr.es/m/ZtesYQ-WupeAK7xK@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5735521ac2d52485bca673039ba43e2b8cc71cd4

Modified Files
--------------
src/interfaces/libpq/Makefile | 2 +-
src/interfaces/libpq/meson.build | 1 -
src/interfaces/libpq/t/005_negotiate_encryption.pl | 7 +++++--
src/test/modules/test_misc/t/005_timeouts.pl | 5 +----
.../modules/test_misc/t/006_signal_autovacuum.pl | 9 +++++++++
src/test/perl/PostgreSQL/Test/Cluster.pm | 22 ++++++++++++++++++++++
src/test/recovery/t/041_checkpoint_at_promote.pl | 5 +----
7 files changed, 39 insertions(+), 12 deletions(-)