Two small bug fixes to t/01-dict.t

Typo in file name that it opens ("dict" => "_dict")

Specify the perl executable in the explanation of how to set the test up
to run, using $^X to give the current executable path.
Added regexp_string() method

This is just a thin wrapper around the _regexp() private method. This
was suggested at https://rt.cpan.org/Ticket/Display.html?id=37287

As per the rt ticket, this is useful if you want to add modifiers (eg.
case insensitivity) to the regular expression produced by Regexp::Trie.