Improve normalise

When reviewing the Shake normalise function, I realised the implementation I had was all wrong. I should backport the appropriate Shake tests to the FilePath library and see if that also needs improvements.

Start by locating the Shake normalise tests and the normalise implementation in the FilePath library. Compare the relevant Shake tests with the existing FilePath tests, then determine which tests can be backported and what improvements they require. Done means the appropriate tests are present and passing, with any needed normalise changes covered.

Written by the indexing model from the issue text.