verilog: fix string literal regular expression (#5187)

* 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).

frontends/verilog/verilog_lexer.l

@@ -372,7 +372,7 @@ TIME_SCALE_SUFFIX [munpf]?s
 }
 
 \"		{ BEGIN(STRING); }
-<STRING>([^\"]|\\.)+	{ yymore(); }
+<STRING>([^\\"]|\\.)+	{ yymore(); }
 <STRING>\"	{
 	BEGIN(0);
 	char *yystr = strdup(YYText());