Disable doctest in coverage

Whilst testing the sbt-scoverage plugin for scala.js on the cats code, I noticed that there was greater test coverage in JVM. Turns out that this is because doctest is currently jvm only and there is some code that is not tested in the real tests, but happen to be called in doctest where the methods are in a comment. E.g. Bifunctor.leftWiden and all of Choice. About 1% of tests are like this.

So doctest should be disabled in coverage, so that the proper test coverage is measured. It could be that the doctest really tests the code, but more likely is just an example of usage.

OTOH.. @andyscott recently had a PR https://github.com/typelevel/cats/pull/1352 with two lines missed

Turns out this a bug in scoverage-scalac-plugin as it currently "detects" macro implementations by looking at methods that return an Expr, and here a Tree is returned. So actually, the whole macro method should be excluded, but currently is included.

Start by comparing JVM and Scala.js coverage behavior for doctest, then inspect the scoverage-scalac-plugin handling described in the issue. Check cats/arrow/FunctionK.scala around line 120 and the related macro coverage behavior. Done means coverage excludes doctest-only calls and correctly excludes the affected macro implementation.