Fix formatting of tables with thead/tfoot but no tbody

Fix formatting of tables with thead/tfoot but no tbody

Correctly handle formatting of tables containing thead and/or tfoot, but
without any tbody, to prevent including thead/tfoot content twice within
the table's derived body.
* lisp/net/shr.el (shr--fix-tbody): Omit 'thead' and 'tfoot' from
implicit body.  (Bug#71685)
* test/lisp/net/shr-resources/table.html:
* test/lisp/net/shr-resources/table.txt: New tests for table rendering.

(cherry picked from commit 9625e4af9946436cea6ba3f41f6c384634bcc999)