test: ensure BytesMut::advance reduces capacity

[camshaft]

This PR adds a test to ensure the behavior of BytesMut::advance is preserved in order to avoid #725.

I also confirmed that this test fails with the change from #698.

---- advance_bytes_mut_remaining_capacity stdout ----
testing capacity=0, len=0, advance=0
testing capacity=1, len=0, advance=0
testing capacity=1, len=1, advance=0
testing capacity=1, len=1, advance=1
thread 'advance_bytes_mut_remaining_capacity' panicked at tests/test_bytes.rs:701:17:
assertion `left == right` failed: Buf::advance should reduce the remaining capacity
  left: 1
 right: 0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    advance_bytes_mut_remaining_capacity

[camshaft]

Looks like my brute force test is quite slow under miri. I think it's probably a good idea to reduce the search space a bit under that mode.