add_test: Revert "Allow special characters in test name"
Revert !4754 (merged). Unfortunately the fix breaks projects that were working around the limitation with manual escaping. The fix can be re-introduced with a policy in a future version.
Also add a 3.18.1 release note explaining the change.
Fixes: #21017 (closed), #20965 (closed)
Issue: #19391 (closed)
Backport: release