test coverage is technically incorrect

[cyphar]

We currently try to generate a report from test coverage files that were produced by binaries build with different feature flags. This is not really correct AFAICS -- the instruction profiling is only meant to work with the binary it was compiled against, and cargo llvm-cov report doesn't support multiple runs in this way. I suspect this is the cause of the

% cargo llvm-cov report
warning: 207 functions have mismatched data
...

warnings we have. #223 will help reduce the problem, but _test_as_root is still going to be required and so the issue will still persist. In theory we could try to remove _test_as_root but there really is a lot of stuff that needs it and (because Rust's test harness is so emaciated) we cannot skip tests in an ergonomic way.