Fixed leading underscore problem in t/vmethods/test.t

The test named "text.split.join d" in t/vmethods/test.t was expecting two leading underscores in the output.  This didn't match actual behavior, and I couldn't find any reason to expect that in a 'join', two underscores would be prepended.  Removing the underscores from the expected output in the test results in a 'PASS'.

This issue was preventing clean install.  I don't know if it was an issue before Perl 5.18, but certainly it was for 5.18.