pgsql: Add tests for low-level PGLZ [de]compression routines

Add tests for low-level PGLZ [de]compression routines

The goal of this module is to provide an entry point for the coverage of
the low-level compression and decompression PGLZ routines.  The new test
is moved to a new parallel group, with all the existing
compression-related tests added to it.

This includes tests for the cases detected by fuzzing that emulate
corrupted compressed data, as fixed by 2b5ba2a0a141:
- Set control bit with read of a match tag, where no data follows.
- Set control bit with read of a match tag, where 1 byte follows.
- Set control bit with match tag where length nibble is 3 bytes
(extended case).

While on it, some tests are added for compress/decompress roundtrips,
and for check_complete=false/true.  Like 2b5ba2a0a141, backpatch to all
the stable branches.

Discussion: https://postgr.es/m/adw647wuGjh1oU6p@paquier.xyz
Backpatch-through: 14

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/42473d90098da256e3bf2bdf793e8b7a35de384b

Modified Files
--------------
src/test/regress/expected/compression_pglz.out | 65 +++++++++++++++++++++++++
src/test/regress/parallel_schedule             |  7 ++-
src/test/regress/regress.c                     | 66 ++++++++++++++++++++++++++
src/test/regress/sql/compression_pglz.sql      | 53 +++++++++++++++++++++
4 files changed, 190 insertions(+), 1 deletion(-)