utils/fs: move 'os' and 'test' to separate packages.

[smola]

• create utils/fs/test package to expose generic test suite to 3rd party fs implementations.
• move 'os' to its own package to avoid cyclic dependency (test -> fs -> test, becomes test -> fs, os -> test, os -> fs).
• remove TestCreateAndWrite: some of our implementations cannot read a file that was just created, written and not closed yet.

[alcortesm]

Why are we moving os type to its own package?

We have been moving os back and forth its own package several times now :), we should make a final decision about it.

[smola]

@alcortesm We moved the generic fs suite to a separate package test so that is can be used by 3rd party fs implementations. Now the os implementation depends on test and test depends on fs, so it is a cyclic dependency and not allowed. So we need to have os in its own package if we want to expose the test package. I'm updating the PR description with this.