Don't use /tmp in tests

Apparently on a mac, getCanonicalPath resolves /tmp to /private/tmp, causing
this test to fail. I've replaced the example with something semantically
equivalent for testing purposes, but it will actually succeed when building
on OSX.
Merge pull request #21 from mikedougherty/fixup-relpath-test

Don't use /tmp in tests, as this can fail on OS X.