Misc improvements to the Gitlab-CI pipeline for MariaDB

- Add new Ninja and Clang build jobs. This helps to ensure those
  toolchains also work in addition to default CMake/gcc.

- Generate dependencies.dot/png to illustrate the CMake/Make/Ninja
  build dependencies. Viewing this image and identifying bottle necks
  in parallelism can help make the build run faster.

- Enable CUnit tests now as they are fixed on 10.6 (MDEV-25820).

- Limit parallel builds to 2 CPUs (full parallelism needs MDEV-25968) on
  CMake/Make. Now only the Ninja builds run full parallel builds as only
  Ninja is smart enough to prevent builds failing on resource
  over-consumption.

- Enable Gitlab-CI cache for job 'centos8' for ccache so that it builds
  faster. Don't use Gitlab-CI cache for other jobs, as it would too easily
  use up all free tier storage on Gitlab.com and force users to get a paid
  account just for MariaDB builds.

- On other jobs clean away ccache, as it only had a 5% hit rate on single
  builds with no downloaded cache.

- Dump full database contents during the test install so that one can
  use diff to compare the database contents at different stages and thus
  track/debug potential bugs in mariadb-install-db and mariadb-upgrade
  code.

Bugfixes:

- Zero out ccache stats before each run so that 'ccache -s' would actually
  show the stats for the latest run.

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