[ADT] BitVector: give `subsetOf(RHS)` name to `!test(RHS)` (NFC) (#170875)
Define `LHS.subsetOf(RHS)` as a more descriptive name for `!LHS.test(RHS)`
and update the existing callers to use that name.
Co-authored-by: Jakub Kuderski <jakub@nod-labs.com>