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

docs/tests/FILEFORMAT.md

@@ -76,6 +76,14 @@ For example, to insert the word hello 100 times:
 
     %repeat[100 x hello]%
 
+## Whitespace
+
+To add significant whitespace characters at the end of the line, or to empty
+lines:
+
+    %spc%
+    %tab%
+
 ## Insert capped epoch days
 
 Mostly to test capped cookie expire dates: `%days[NUM]` inserts the number of