Test that UTF-8 in the look-ahead of (?(?=...)...) restarts the sizing parse.

Test that UTF-8 in the look-ahead of (?(?=...)...) restarts the sizing parse.

S_reg() recurses to itself to parse various constructions used as the
conditionals in conditional matching. Look-aheads and look-behinds can turn
out to need to be sized as UTF-8, which can cause the inner S_reg() to use
the macro REQUIRE_UTF8 is used to restart the parse. Test that this is
handled correctly.