[MC/DC] Introduce `class TestVector` with a pair of `BitVector` (#82174)

This replaces `SmallVector<CondState>` and emulates it.

- -------- True  False DontCare
- Values:  True  False False
- Visited: True  True  False

`findIndependencePairs()` can be optimized with logical ops.

FIXME: Specialize `findIndependencePairs()` for the single word.