Merge branch 'skia/ci_speedup' into 'master'

CI speedup

* Put the Xapian search index in `/dev/shm`, which is an in-memory storage makes the tests go from about 1500s to about 600s.
* Keep the `pip` cache between jobs, to avoid re-downloading all the wheels all the time. This gains about 1min.

See merge request ae/Sith!279

.gitlab-ci.yml

@@ -4,19 +4,27 @@ stages:
 test:
   stage: test
   script:
+  - env
   - apt-get update
   - apt-get install -y gettext python3-xapian libgraphviz-dev
   - pushd /usr/lib/python3/dist-packages/xapian && ln -s _xapian* _xapian.so && popd
   - export PYTHONPATH="/usr/lib/python3/dist-packages:$PYTHONPATH"
   - python -c 'import xapian' # Fail immediately if there is a problem with xapian
-  - pip install -r requirements.txt
-  - pip install coverage
+  - pip install -U -r requirements.txt
+  - pip install -U coverage
+  - mkdir -p /dev/shm/search_indexes
+  - ln -s /dev/shm/search_indexes sith/search_indexes
   - ./manage.py compilemessages
   - coverage run ./manage.py test
   - coverage html
   - coverage report
   - cd doc
   - make html # Make documentation
+  variables:
+    PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip_tests"
+  cache:
+    paths:
+      - .cache/pip_tests
   artifacts:
     paths:
       - coverage_report/
@@ -24,5 +32,10 @@ test:
 black:
   stage: test
   script:
-    - pip install black
+    - pip install -U black
     - black --check .
+  variables:
+    PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip_black"
+  cache:
+    paths:
+      - .cache/pip_black