codespell: Ignore test output regexes

Ignore RunCMake output regexes (i.e. *-stdout.txt, *-stderr.txt) when
checking spelling. Because these are regular expressions, they have a
higher likelihood of having false positives, and because they are
expected output, any text they contain ought to also appear in whatever
code generates it (or else, is probably text that depends on user input
and shouldn't be spell-checked anyway).

.codespellrc

@@ -20,6 +20,8 @@ skip =
   Tests/StringFileTest/test.utf8,
   Utilities/cm*,
   *.pfx,
+  *-stdout.txt,
+  *-stderr.txt
 
 # noqa: spellcheck off
 ignore-words-list =