test_tidstore: Stabilize regression tests by sorting offsets.

test_tidstore: Stabilize regression tests by sorting offsets.

TidStoreSetBlockOffsets() requires its offsets array to be strictly
ascending and asserts this precondition. In test_tidstore, we were
passing random offset numbers deduplicated by a DISTINCT clause in an
array_agg() call directly to the do_set_block_offsets() test
harness. However, DISTINCT without an ORDER BY clause does not
guarantee sorted results according to the SQL standard.

Fix this by sorting the offsets in-place inside do_set_block_offsets()
before calling TidStoreSetBlockOffsets().

While this assertion failure is not observed during regular regression
tests because they use queries simple enough that the optimizer
consistently chooses plans yielding sorted results, it makes sense to
stabilize the test. The failure could theoretically occur depending on
the optimizer's plan choice, and has been reported when experimenting
with certain third-party extensions.

Backpatch to v17, where test_tidstore was introduced, to ensure
extension development on stable branches does not hit this assertion.

Reported-by: Andrei Lepikhov <lepihov@gmail.com>
Author: Andrei Lepikhov <lepihov@gmail.com>
Discussion: https://postgr.es/m/b97f1850-fc7b-43c4-9b04-4e97bb9e7dc0@gmail.com
Backpatch-through: 17