.gitlab-ci.yml: Assert that tests without `usedevelop` work

This is partially to prevent issues like #1121, but also to ensure our
manifests can be packaged correctly.

For now only the test suite is exercised, in the future it would be
nice to expand this to other tests (such as the man pages, or source
tests), but those don't have -nocover versions yet.
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index ef31ad3..5576a44 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -84,6 +84,16 @@
   only:
   - schedules
 
+tests-isolated:
+  # Ensure that our tests also function without usedevelop, although
+  # this forces us to disable coverage reports
+  image: registry.gitlab.com/buildstream/buildstream-docker-images/testsuite-fedora:29-master-85428307
+  variables:
+    TEST_COMMAND: "tox -e py{35,36,37}-nocover -- ${PYTEST_ARGS}"
+    EXTERNAL_TESTS_COMMAND: "tox -e py{35,36,37}-external-nocover -- ${PYTEST_ARGS}"
+  <<: *tests
+
+
 tests-unix:
   # Use fedora here, to a) run a test on fedora and b) ensure that we
   # can get rid of ostree - this is not possible with debian-8