Wait for merge error message in file_locking_spec:63

What does this MR do?

Adds wait for error message in MR. Previously test failed while merge was in progress(which took quite some time) and no error message yet appeared.

This test was quarantined due to a code bug: #43105 (closed)

Closes #33118 (closed)

Screenshots

Adds wait for failed to merge message:

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content