Cat /proc/partitions and list /dev in GitLab CI test jobs (!113)

Add these simple debugging aids to the GNOME GitLab CI test job.
They've been needed before [1] so add them permanently.

[1] 57983b9fc2
    Create block special devices needed by test_BlockSpecial in GitLab
    CI jobs (!59)
        "Contents of /proc/partitions inside the Docker image when this
        test CI job failed:
        ...
        And the listing of /dev/:
        "

Closed !113 - Fix occasional GitLab CI test jobs failures on
              BlockSpecial unit tests

.gitlab-ci.yml

@@ -80,6 +80,8 @@ stages:
     - export GTEST_FILTER=`tests/exclude_loopdev_tests.sh tests/test_SupportedFileSystems.cc`
     - echo $GTEST_FILTER
     - fgrep -v nodev /proc/filesystems | sort
+    - cat /proc/partitions
+    - ls -l /dev
     # Create needed /dev entries for unit tests in Docker images.
     - tests/makedev.sh
     - make check