Integrate assertions in kext & mock code with XCTest assertions

Assertions in the kext unit testing environment have various problems at the moment:

• assert()s in the kext currently don't do anything in the testable kext target. It should instead be possible to verify in a unit test that an assert() in the kext code under test failed.
• assert()s in the mock code (vnode, mount, etc.) use the platform/stdlib assert implementation and thus crash the test runner when they fail.
• The XCTAssert* macros don't work outside instance methods of the test class.
• The XCTAssert* macros don't terminate the current test when they fail, thus making it likely that the rest of the failing test will trigger undefined behaviour (dereferencing null pointers that were asserted to be non-null, etc.)
• Related: The XCTest framework really isn't designed for testing C++ code and assumes various Objective-C-isms.

The above problems have similar or highly related solutions, so it makes sense to track them under the umbrella of one issue.

Start by examining the testable kext target and the assertion paths in the kext and mock code, including the vnode and mount mocks. Review how XCTest assertions are used outside test instance methods and determine the expected behavior when each assertion fails. Done means the listed assertion cases are handled without crashing the test runner and are verifiable from unit tests.