Figure out a way to incorporate trybuild runs in tarpaulin reports

[jawoznia]

          > Only one thing bothers me - why the code coverage is not calculated properly here?

I think this is because trybuild invokes rustc on it's own per test file (otherwise tests would fail the original process run by us).
Then it makes sense that tarpaulin doesn't incorporate those runs into it's report since those are separate processes.

It's sad as I thought we will be able to cover error cases using trybuild.
Solution to this is not obvious to me at the moment so I will open a separate issue to track it.

Originally posted by @jawoznia in #338 (comment)