testsuite: Provide means to regexp in multiline patterns

testsuite: Provide means to regexp in multiline patterns

Those multi-line-patterns are literal.  Sometimes a regexp
needs to be matched.  This is a start: just three elements
are supported: "(" ")" and the compound ")?" (and on second
thought, it can be argued that "(...)" alone is not useful).
Note that Tcl "string map" is documented to have the desired
effect: a once-over but no re-recognitions of previously
replaced mapped elements.  Also, drop a doubled "containing".

testsuite:
	* lib/multiline.exp (_build_multiline_regex): Map
	"{re:" to "(", similarly ")?" from ":re?}" and the
	same without question mark.