[CI] Optional tests for expensive tests.

This adds build that runs expensive tests, including Vc on with at least AVX.
The build is allowed to fail to compensate for the possibility of no runner
beeing available.

.gitlab-ci.yml

@@ -2,6 +2,18 @@
 variables:
   DUNECI_TEST_LABELS: quick
 
+debian:10--gcc--expensive:
+  # This image has Vc
+  image: duneci/base:10
+  script: duneci-standard-test
+  # allow expensive tests
+  variables: {DUNECI_TEST_LABELS: ""}
+  # require AVX to properly test Vc
+  tags: [duneci, "iset:avx"]
+  # allowed to fail to e.g. do no hold up a merge when a runner supporting avx
+  # is unavailable
+  allow_failure: true
+
 debian:10  gcc:c++17:
   image: duneci/base:10
   script: duneci-standard-test