ci: prevent multithreading for subplot tests

- There is only one virtual card, accessing it from multiple programs
    in parallel leads to undefined behavior.

.gitlab-ci.yml

@@ -149,7 +149,7 @@ subplot:
   script:
     # make sure a virtual card is available, so that the subplot tests are
     # generated
-    - CARD_BASED_TESTS=true cargo test
+    - CARD_BASED_TESTS=true cargo test -- --test-threads 1
   cache:
     # inherit all general cache settings
     <<: *general_cache_config

tools/cargo-test-in-docker

@@ -23,4 +23,4 @@ su - -c "sh /home/jcardsim/run-card.sh >/dev/null" jcardsim &&
 cd /src/tools &&
 CARGO_TARGET_DIR=/cargo/ cargo update &&
 CARGO_TARGET_DIR=/cargo/ cargo build -vv &&
-CARGO_TARGET_DIR=/cargo/ cargo test'
+CARGO_TARGET_DIR=/cargo/ cargo test -- --test-threads 1'