Use tracefiles for coverage

[goerz]

This is to avoid .cov files cluttering the src directory.

I'm opening this PR to test how it runs in the CI and whether the coverage information is correctly submitted to Codecov

[goerz]

For clarification: This uses a single tracefile.info when testing locally (via make test or by running test() in the development REPL). On CI, the more traditional --code-coverage="user" is still used, which produces .cov files in the src folder. I got the impression the julia-actions/julia-processcoverage works better with (only with?) .cov files. Also, we don't really care about "littering" on the CI system.

[IanButterworth]

I recommend not using --code-coverage="user" but instead use the --code-coverage=@{path to repo} because user tests all non-Base code, including stdlibs. @ path approach will be much faster

[IanButterworth]

I forgot to mention it won't work on 1.6, so you may want to pass user or @ in via a matrix or something

[goerz]

Yeah, I've been trying that, but Github apparently doesn't allow using matrix values either inside the shell: or if: values for a step.

[goerz]

Well, splitting it into two jobs (test and testnocov) does the trick.

I'm going to get rid of macOS testing, too. It fails (hangs) about 10% of the time, and it's really not necessary. First, I'm developing on macOS, so it's getting plenty of testing. Second, I've never seen something work on Linux and fail on macOS.