MDEV-39828 Stabilize query_cache_executable_comments counter checks

[tonychen2001]

Description

The test main.query_cache_executable_comments is unreliable and fails on
the builders (MDEV-39828).
It asserts the absolute values of the global status counters Qcache_inserts
and Qcache_hits, but resets them per case with FLUSH STATUS.

FLUSH STATUS does not reset the global counters query_cache.hits /
query_cache.inserts. As a result the recorded values were
actually cumulative from server start, and the test only passed against a
pristine mysqld. When the server instance is reused with prior query-cache
activity (server reuse, --repeat), the leftover counter values are
added to every assertion, producing a constant offset and a "Result length
mismatch".

Fix: Replace the FLUSH STATUS calls with FLUSH GLOBAL STATUS which actually resets the Qcache counters. This is the idiomatic pattern
already used by every other query-cache test (query_cache.test,
query_cache_with_views.test, subselect_cache.test, the query_cache*.inc
includes, etc.).

How can this PR be tested?

Execute the main.query_cache_executable_comments test in mysql-test-run.

Before this change:

The test passes in isolation but fails whenever the server is reused with
prior query-cache activity. This is reproducible with --repeat=2 (the
second iteration reuses the same mysqld instance):

@@ -30,10 +30,10 @@
 SELECT /*!50300 c1 */ FROM t1;
 SHOW STATUS LIKE 'Qcache_inserts';
 Variable_name	Value
-Qcache_inserts	1
+Qcache_inserts	7
 SHOW STATUS LIKE 'Qcache_hits';
 Variable_name	Value
-Qcache_hits	1
+Qcache_hits	3
 SHOW STATUS LIKE 'Qcache_queries_in_cache';
 Variable_name	Value
 Qcache_queries_in_cache	1
...
Result length mismatch

After this change:

Running the test with --repeat=2 in
mysql-test-run shows no failures, with the server reused throughout:

==============================================================================

TEST                                      RESULT   TIME (ms) or COMMENT
--------------------------------------------------------------------------

worker[01] Using MTR_BUILD_THREAD 300, with reserved ports 19000..19029
worker[01] mysql-test-run: WARNING: running this script as _root_ will cause some tests to be skipped
main.query_cache_executable_comments     [ pass ]    108
main.query_cache_executable_comments     [ 2 pass ]     28
--------------------------------------------------------------------------
The servers were restarted 0 times

Completed: All 2 tests were successful.

Basing the PR against the correct MariaDB version

• This is a bug fix, and the PR is based against the branch 13.0.

Copyright

All new code of the whole pull request, including one or several files that are either new files or modified ones, are contributed under the BSD-new license. I am contributing on behalf of my employer Amazon Web Services, Inc.

[gemini-code-assist]

Code Review

This pull request updates the query cache executable comments test suite by replacing FLUSH STATUS with FLUSH GLOBAL STATUS in both the test and result files. This change also updates the expected status variable values, such as Qcache_inserts and Qcache_hits, in the test results. I have no feedback to provide as there are no review comments.

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