Support runner config

[QuarticCat]

Cargo has a configuration target.<triple>.runner that allows you to specify a runner program to take over the execution of compiled binaries. It seems like Tarpaulin doesn't support it. Here's an example:

Create runner.sh with content:

#!/bin/bash -e
echo 123
"$@"

Then execute:

$ cargo test
...
$ export CARGO_TARGET_AARCH64_APPLE_DARWIN_RUNNER=./runner.sh
$ cargo test
...
123
...
$ cargo tarpaulin
...

In my specific case, I use a runner to send binaries to a remote machine and execute them there.

[xd009642]

So this is a bit more complex, but if you're running coverage on a remote machine how will tarpaulin get the coverage for it? 👀

The only way I can see that one working is if you add a step to the runner afterwards to download all of the profraw files off the machine into the tarpaulin profraw folder. But it'll still be a bit fragile (i.e. things like ptrace won't work, other things may struggle depending on what the runner does)

I can do the naive implementation easily enough (just run the runner and hope the user has sorted out those finer details), but I'll probably have to think a bit about documentation/DX before shipping it 🤔

[QuarticCat]

Ah... That's quite a problem. I guess I'd better ignore those tests that only work remotely.