[ci] Try removing the test prefix before starting a new test cycle

.gitlab-ci.yml

@@ -10,6 +10,7 @@ py27-linux:
   - cd bin && ln -s ../src/cpulimit/src/cpulimit . && cd -
   - export COVERAGE_FILE=.coverage.django
   - export BEAT_TEST_PREFIX=/var/tmp/test_prefix
+  - rm -rf $BEAT_TEST_PREFIX
   - ./bin/coverage run --source='./beat/web' ./bin/django test --settings=beat.web.settings.test -v 2
   - export BEAT_CMDLINE_TEST_PLATFORM=django://beat.web.settings.test
   - export COVERAGE_FILE=.coverage.cmdline