Currently in order to test this you'll need this branch of Dotty.
There is an sbt alias that you're able to use, testCoverage which will turn on
coverage, run the tests, and generate your report. You'll then be able to find
the report in
./target/scala-3.1.2-RC1-bin-SNAPSHOT/scoverage-report/index.html.
This is using sbt-scoverage v2.0.0-M1 which will correctly set the -coverage
flag for your project when you turn coverage on, and also pass in the output
dir. You can see if if you open sbt and do a coverge followed by a show Compile / compile / scalacOptions.