Support running the tests on Mac

[cmichi]

We are using the psutil crate to assert that a canvas node process is runnning. This happens here.

On Mac this is not supported though, resulting in a panic:

---- tests::multisig_plain::multisig_works_with_payable_transaction stdout ----
thread 'main' panicked at 'not yet implemented', ~/.cargo/registry/src/github.com-1ecc6299db9ec823/psutil-3.2.1/src/process/sys/macos/process.rs:103:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

We should investigate using either a different crate or a different function from the psutils crate. I would try the latter first, there are a number of functions supported for Mac, just not the one we're using atm.