test/rgw/test_posix_bucket_cache: structure tests with fixture

before this change, we use individual tests to setup/teardown
the fixtures. this renders it difficult to run a certain test.

in this change, we structure the tests with fixtures, so that
we can run any test without worrying the dependencies between
them.

Signed-off-by: Kefu Chai <[email protected]>

Author: tchaikov