Run `meson test` on GitLab CI

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

+image: debian:buster-slim
+
+before_script:
+  - apt update
+  - apt-get install --no-install-recommends -y
+      build-essential
+      meson
+      libnss3-dev
+      sharness
+      libnss-wrapper
+
+test:
+  script:
+    - meson _build/ .
+    - cd _build/
+    - ninja
+    - meson test -v
+  artifacts:
+    when: always
+    paths:
+      - _build/meson-logs
+      - _build/test-results