tests: replace significant invisible spaces with macros

tests: replace significant invisible spaces with macros

To make them explicit, visible, avoid being accidentally trimmed.
Also prevents Git warnings, e.g. on `git am`.

Also:
- runtests: add support for `%spc%` and `%tab%` macros.
- test59: delete non-significant line-ending space.
- spacecheck.pl: drop line-ending whitespace exception for tests.

Closes #19300