* verilog: fix string literal regular expression.
A backslash was improperly quoted, causing string literal matching
to fail when the final token before a closing quote was an escaped
backslash.
* verilog: add regression test for string literal regex bug.
Test for bug triggered by escaped backslash immediately before
closing quote (introduced in ca7d94af and fixed by 40aa7eaf).
a519390fc4