[ADD] mergebot: support for coverage during tests

Runs the test instances of Odoo using `coverage` in parallel mode.

- useful for finding out under-tested parts of the code
- because it only instruments mergeport/forwardport, and the tests do
  so much IO, the wallclock performance impacts are minimal (~2%
  increase with branch coverage analysis, for an increase in CPU
  of ~20%, for the entire testsuite)
- for reporting, the scattered coverage reports need to be aggregated
  using `coverage combine`, followed by rendering with `coverage
  html`, these work out of the box, no parameterization is necessary
- coverage does not run on the test suite, only the modules under test

conftest.py

@@ -76,6 +76,7 @@ def pytest_addoption(parser):
     parser.addoption('--addons-path')
     parser.addoption("--no-delete", action="store_true", help="Don't delete repo after a failed run")
     parser.addoption('--log-github', action='store_true')
+    parser.addoption('--coverage', action='store_true')
 
     parser.addoption(
         '--tunnel', action="store", type="choice", choices=['', 'ngrok', 'localtunnel'], default='',
@@ -372,7 +373,13 @@ def server(request, db, port, module, dummy_addons_path, tmpdir):
         request.config.getoption('--addons-path'),
         dummy_addons_path,
     ]))
+
+    cov = []
+    if request.config.getoption('--coverage'):
+        cov = ['coverage', 'run', '-p', '--source=odoo.addons.runbot_merge,odoo.addons.forwardport', '--branch']
+
     p = subprocess.Popen([
+        *cov,
         'odoo', '--http-port', str(port),
         '--addons-path', addons_path,
         '-d', db,