Set location of coverage file in Travis

.travis.yml

@@ -47,6 +47,12 @@ env:
   - CYLC_TEST_RUN_PLATFORM=false
   # Custom diff command to ignore Xlib errors (xvfb has not RANDR extension).
   - CYLC_TEST_DIFF_CMD="diff -I Xlib -u"
+  # As we fork processes, we need to make sure to point where each forked process
+  # will put its .coverage... files. Otherwise they may be created under directories
+  # that `coverage combine` isn't expecting, causing the report to omit tests.
+  # On the other hand, changing .coveragerc directly affects developers running
+  # coverage locally. Therefore, we have this sed line here only for Travis.
+  - sed -i -e 's|data_file=.coverage|data_file=${TRAVIS_BUILD_DIR}/.coverage|g' .coveragerc
   - COVERAGE_PROCESS_START="${TRAVIS_BUILD_DIR}/.coveragerc"
   matrix:
   - CHUNK="1/4"