warnings: be less picky about occurrences of m4_/b4_ in the output

Reported by Marko Mäkelä.
<https://lists.gnu.org/r/bug-bison/2021-10/msg00026.html>

* src/scan-skel.l: It is ok to have foob4_ or foom4_.
* tests/skeletons.at (Suspicious sequences): New.
3 files changed