fstests: Relax MimeType support checking #5587

Before this change we checked that features.ReadMimeTime was set if
and only if the Object.MimeType method was implemented.

However this test is overly general - we don't care if Objects
advertise MimeType when features.ReadMimeTime is set provided that
they always return an empty string (which is what a wrapping backend
might do).

This patch implements that logic.
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