Add `-Zassert-incr-state` flag to incremental tests

[pierwill]

It'd be good to add these flags to rustc-perf's incr tests once this flag is implemented, I think, FWIW.

Originally posted by @Mark-Simulacrum in https://github.com/rust-lang/rust/issues/85864#issuecomment-851587323

For examples in the rust test suite, see here and here.