Resolve conflict between the fusefs(5) and mac_bsdextended(4) tests

mac_bsdextended(4), when enabled, causes ordinary operations to send many
more VOP_GETATTRs to file system. The fusefs tests expectations aren't
written with those in mind. Optionally expecting them would greatly
obfuscate the fusefs tests. Worse, certain fusefs functionality (like
attribute caching) would be impossible to test if the tests couldn't expect
an exact number of GETATTR operations.

This commit resolves that conflict by making two changes:

1. The fusefs tests will now check for mac_bsdextended, and skip if it's enabled.
2. The mac_bsdextended tests will now check whether the module is enabled, not merely loaded. If it's loaded but disabled, the tests will automatically enable it for the duration of the tests.

With these changes, a CI system can achieve best coverage by loading both
fusefs and mac_bsdextended at boot, and setting
security.mac.bsdextended.enabled=0