test: fix component printing on windows

Commit 084b2ec1 ("test: include component in tap output") introduced
an in hindsight glaringly obvious but fortunately not very critical
Windows-specific bug by failing to take the path separator into account.
This commit rectifies that, the prefix is now correctly stripped.

PR-URL: https://github.com/nodejs/node/pull/6915
Refs: https://github.com/nodejs/node/pull/6653


Reviewed-By: Anna Henningsen <anna@addaleax.net>