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.

cfg.mk

@@ -121,7 +121,7 @@ sc_tests_list_consistency:
 	  cd $(top_srcdir);						\
 	  $(SHELL) build-aux/vc-list-files tests			\
 	    | grep -Ev '^tests/(factor/(run|create-test)|init)\.sh$$'	\
-	    | grep -E "$$test_extensions_rx\$$";			\
+	    | grep -E "($$test_extensions_rx)$$";			\
 	} | sort | uniq -u | grep . && exit 1; :
 
 # Ensure that all version-controlled test scripts are executable.