Fix Zarr 'number of requests' test

[ianhi]

Fixess the test test failures introduced by #10469

That PR introduced a new test that used the time_unit fixture which is scoped to module. This was somehow resulting in leakage of state across tests. Normally this doesn't seem to matter (e.g. other fixtures also use that scope) but it was affecting the get/set counting.

See also: https://docs.pytest.org/en/stable/how-to/fixtures.html#fixture-scopes

This also reverts: 8d2e563 to restore the initial values

@maxrjones your bisecting was very helpful! also @TomNicholas

• Closes #xxxx
• [NA] Tests added
• [NA] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [NA] New functions/methods are listed in api.rst

[TomNicholas]

Well done!!

[ianhi]

I'm still pretty confused as to how this was contaminating relevant between the tests (seemingly zarr_format) but worth noting that "function" is the default scope so we shoudl consider changing the datatree fixture to a smaller scope

[spencerkclark]

Nice catch!