test-require-resolve: use case insensitive compare

This is an alternative fix for the issue that f6e5740 had intended to fix.
It's needed because f6e5740 has been reverted in e24fa83.
See also https://github.com/node-forward/node/issues/20 and #100

@indutny or @bnoordhuis, can I get your review on this?

Because node-forward is closed, here's the text:

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The patch joyent/node@a05f973 was landed to make some tests pass, but I can't really reconstruct what the motivation was. What I can tell is that the test failure wasn't really related to path.normalize() and path.resolve() [not] changing the drive letter case.

This patch has caused a lot of issues, including joyent/node#7031 and joyent/node#7806 and lots of bikeshedding about whether uppercase is the right case or lowercase.

I propose reverting the offending patch, and just doing what node does everywhere else (which is: path functions don't touch the case) and take another look at the cause of those test failures.