Integrate coverage testing

[wilzbach]

Generate Coverage report

This should nearly at the end as it messes up the build repositories. It should be something similar to:

stage ('Test coverage') {
   def repo_name = /python3 -c "import sys, json; print(json.load(sys.stdin)['repository']['name']).toupper()"/
   switch (repo_name) {
      case 'dmd':
          sh 'mv src/dmd src/host_dmd'
          sh 'make -j$N -C dmd/src -f posix.mak MODEL=$MODEL HOST_DMD=./host_dmd clean'
          sh 'make -j$N -C dmd/src -f posix.mak MODEL=$MODEL HOST_DMD=./host_dmd dmd.conf'
          sh 'make -j$N -C dmd/src -f posix.mak MODEL=$MODEL HOST_DMD=./host_dmd ENABLE_COVERAGE=1'
          sh 'make -j$N -C dmd/test MODEL=$MODEL ARGS="-O -inline -release" DMD_TEST_COVERAGE=1'
          sh 'mv src/host dmd src/dmd'
          break;
      case 'druntime':
          sh 'TEST_COVERAGE="1" make -j$N -C . -f posix.mak MODEL=$MODEL unittest-debug'
          break;
      case 'phobos':
          sh 'ENABLE_COVERAGE="1" make -f posix.mak MODEL=$MODEL unittest-debug'
          // tests needs to be run separately due to https://issues.dlang.org/show_bug.cgi?id=16397
          sh 'make -f posix.mak $(find std etc -name "*.d" | sed "s/[.]d$/.test")'
          break;
      default:
   }

CodeCov

Assuming the CodeCov tokens (can be fetched at e.g. https://codecov.io/gh/dlang/phobos/settings) are loaded in the environment something as CODECOV_TOKEN_PHOBOS (and similarly for dmd and druntime) like this should work:

stage ("Upload coverage report") {
    def repo_name = /python3 -c "import sys, json; print(json.load(sys.stdin)['repository']['name']).toupper()"/
    sh 'curl -s https://codecov.io/bash | bash -s - -t CODECOV_TOKEN_${DMD}'
}

CodeCov uptime doesn't seem to be perfect, we might want to provide a fallback URL as well - see e.g.
dlang/phobos#5140

(though I am not sure what's the proper behavior as the CodeCov API is then probably down as well)

[MartinNowak]

That might actually be some work because external testers usually require authentication with coverage services.

[wilzbach]

Only the CodeCov token would be required.
However at the moment we don't run any of the DMD, druntime or Phobos testsuites and this requires quite some resources.
CircleCi does a good job at this and we even use three parallel runs at DMD now: https://github.com/dlang/dmd/blob/master/.circleci/config.yml (x86, x86_64, x86_64 - pic).
They are all required to improve the overall coverage (see dlang/dmd#7818).
If we ever want to unify this, it's probably easier to move the .circleci/ci.sh script to tools and fetch it from each repo.