gh-69113: Fix doctest to report line numbers for __test__ strings (#141624)

Enhanced the _find_lineno method in doctest to correctly identify and
report line numbers for doctests defined in __test__ dictionaries when
formatted as triple-quoted strings.

Finds a non-blank line in the test string and matches it in the source
file, verifying subsequent lines also match to handle duplicate lines.

Previously, doctest would report "line None" for __test__ dictionary
strings, making it difficult to debug failing tests.

Co-authored-by: Jurjen N.E. Bos <jneb@users.sourceforge.net>
Co-authored-by: R. David Murray <rdmurray@bitdance.com>