Fix pilcom proof fail test functions

[qwang98]

Depends on #2535 and #2557. Split assert_proofs_fail_for_invalid_witnesses_pilcom into assert_proofs_fail_for_invalid_witnesses_pilcom_composite and assert_proofs_fail_for_invalid_witnesses_pilcom_monolithic. These two APIs are needed because we might generate different witnesses for pilcom composite vs pilcom monolithic under two scenarios:

1. The two backends can have different witnesses to start with. This hasn't been an issue because most of our test cases are very simple machines.
2. After updates in PoC for two passes of pilopt #2535, if we eventually implement different optimizations for the two different backends, they might compute two different witnesses.