test: Clean up skipped tests

- Remove obsolete circular dependency test (not applicable in new architecture)
- Remove placeholder mounting tests (actual mounting tested elsewhere)
- Remove all mbro integration tests (high maintenance, low value)
- Keep conditional skips for internet/platform-specific tests
- Keep API exploration tests that document unsupported features

This reduces test maintenance burden while keeping valuable tests.

Signed-off-by: Phillip Sitbon <phillip.sitbon@gmail.com>

Author: sitbon