tests: fs: Ensure file_path includes max file name length

A hardcoded path lenth of 80 will not be able to suppor the full length
of 255 when LFN is enabled. This does produce a compiler error,
thankfully, this is only applicable to the test cases.

Signed-off-by: Roman Vaughan <nzsmartie@gmail.com>
Signed-off-by: David Brown <david.brown@linaro.org>