Refactored tests to allow setup_lexer to instantiate a new lexer every time. Not ready to flip switch yet.
Changed assert_lex to pull out string token if array comes back for yacc_value.

[git-p4: depot-paths = "//src/ruby_parser/dev/": change = 8758]
assert_lexer now runs with fresh parser/lexer each time.
Fixes two false negatives. 

[git-p4: depot-paths = "//src/ruby_parser/dev/": change = 8759]