maint: improve ERE in sc_tests_list_consistency

maint: improve ERE in sc_tests_list_consistency

The ERE used lacks the grouping of the extensions and therefore would
also match files where the first two patterns are not at the end of
the line:
  grep -E '\.sh|\.pl|\.xpl$'

* cfg.mk (sc_tests_list_consistency): Add grouping (...) around the
sub-patterns.  While at it, also remove the redundant escaping, i.e.,
\$$ -> $$ to be consistent with the rest of this file.