add_test: Revert "Allow special characters in test name"

Revert commit f84af8e270 (add_test: Allow special characters in test
name, 2020-05-16, v3.18.0-rc1~142^2).  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, #20965
Issue: #19391
59 files changed