Prior to this change, this test was failing for me on a new windows
machine, with mismatching drive letters:
helpers_test.go:20: (C:/Users/Foo Bar/cli/altsrc/flag_test.go:195) Expected C:\path\to\source\hello (type string) - Got D:\path\to\source\hello (type string)
Rather than hard-coding the drive letter in the expected path, we can
expect the filepath.Abs'ed version.
This should fix part of #1105.
9eb96e77a1