TST: Make HDF5 fspath write test robust
The test_write_fspath_all test would fail on the HDF5 example
occasionally (about 1/100 in my experience). Apparently you don't get an
identical HDF5 every single time. This refactors that test out to its own where
we write and read both versions, and compare equality that way.

See https://github.com/pandas-dev/pandas/pull/14026#issuecomment-305554714 and https://circleci.com/gh/pandas-dev/pandas/2507?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link for an example