WAExternalFileLibraryTest>>#testFileNameFor does not account for case insensitive filesystems

[jbrichau]

The GRPlatform>>fileNameFor: method essentially validates and "normalizes" a filesystem path string. On case insensitive file systems, the string that is returned may differ in case (e.g. in Windows, the drive letter casing can be different). The test now fails where this happens while it should not.