[Fix] stack trace path parsing on windows

Source files which are located in the current working directory, have a part of their path replaced by `path.sep + '$CWD'`. However, this causes the regular expression for stack trace lines to not match on windows.

Example of a stack trace line, after replacement (`console.log(lineWithTokens)` in lib/test.js):

```
   at Test.assert [as _assert] (\$CWD\example\my-test.js:483:11)
```

The part of the regexp that fails is `(?:\/|[a-zA-Z]:\\)`. I fixed this by allowing the path to start with a backslash. So instead of `\/`, the regexp uses `[/\\]`.

This issue is already covered by existing test cases that are currently failing when they are ran on windows. For example: `.\node_modules\.bin\tap test/*.js`

lib/test.js

@@ -518,9 +518,9 @@ Test.prototype._assert = function assert(ok, opts) {
                 Last part captures file path plus line no (and optional
                 column no).
 
-                    /((?:\/|[a-zA-Z]:\\)[^:\)]+:(\d+)(?::(\d+))?)\)?/
+                    /((?:[/\\]|[a-zA-Z]:\\)[^:\)]+:(\d+)(?::(\d+))?)\)?/
             */
-			var re = /^(?:[^\s]*\s*\bat\s+)(?:(.*)\s+\()?((?:\/|[a-zA-Z]:\\)[^:)]+:(\d+)(?::(\d+))?)\)?$/;
+			var re = /^(?:[^\s]*\s*\bat\s+)(?:(.*)\s+\()?((?:[/\\]|[a-zA-Z]:\\)[^:)]+:(\d+)(?::(\d+))?)\)?$/;
 			// first tokenize the PWD, then tokenize tape
 			var lineWithTokens = $replace(
 				$replace(