fix(tests): escape the regex in test_verify_pdf's docstring and compile-check tests/*.py (#503)

FindNonAsciiDateRangesTests' docstring (from #498) quotes the regex `\s*`
in a non-raw string. `\s` is an invalid escape sequence, so Python 3.12+
prints a SyntaxWarning for it (CPython gh-98401) when
tests/test_verify_pdf.py is compiled. It is in this repo's own CI log
for a45bbd1 on 3.12, 3.13 and 3.14, at tests/test_verify_pdf.py:75, while
the run stays green. Under `-W error::SyntaxWarning` the module fails to
import and the whole test_verify_pdf suite drops out of the run.

Escaped in the docstring. ToolsCompileWithoutWarnings (#480) only compiled
tools/*.py, which is why this got through; it now compiles tests/*.py too.
With the unescaped docstring it fails naming tests/test_verify_pdf.py, and
it passes with the fix.

Verification on Python 3.10, 3.11, 3.12, 3.13 and 3.14, from a clean
bytecode cache: `python -W error::SyntaxWarning -m unittest discover -s
tests -t .` OK (501 tests, the same count as master),
`tools/lint_skills.py`, `tools/check_framework_version.py`,
`tools/security_guards.py`.

tests/test_verify_pdf.py

@@ -72,7 +72,7 @@ class FindNonAsciiDateRangesTests(unittest.TestCase):
     The fold that makes `--contains "2016-2024"` pass on that layer (#458) is
     what makes `--contains` unable to detect it - so this check never folds.
 
-    The newline cases are yang2632's finding on the PR: `\s*` between year and
+    The newline cases are yang2632's finding on the PR: `\\s*` between year and
     dash also matched a line break, so a heading ending in an ASCII date joined
     a dash that merely opened the next line.
     """