test: add TAP tests and housekeeping for mysql-zstd_compression_level

- New test mysql-zstd_compression_level-t.cpp: validates default value,
  range enforcement (1-22), rejection of 0 and 23, independence from
  mysql-protocol_compression_level, and functional compressed connection.
- Register mysql-zstd_compression_level-t in groups.json (same groups as
  mysql-protocol_compression_level-t).
- Add variable visibility check to reg_test_2793-compression-t.cpp.
- Add unit test binary pattern to .gitignore.

Author: renecannao

.gitignore

@@ -211,5 +211,9 @@ scripts/mcp/DiscoveryAgent/ClaudeCode_Headless/tmp/
 test/tap/tests/test_cluster_sync_config/test_cluster_sync_nomonitor/cluster_sync_node_stderr.txt
 test/tap/tests/test_cluster_sync_config/test_cluster_sync_withmonitor/cluster_sync_node_stderr.txt
 
+# unit test binaries (built but not tracked)
+test/tap/tests/unit/*-t
+
 # test-scripts runtime dependencies (created by run-tests-isolated.bash)
 test-scripts/deps/
+.worktrees/