experiment: Remove --parallel test execution

Author: simonjbeaumont

docker/docker-compose.yaml

@@ -44,7 +44,7 @@ services:
 
   compatibility-test: &HACK
     <<: *common
-    command: /bin/bash -xcl "cat /proc/cpuinfo && cat /proc/meminfo && swift test --parallel --num-workers $$(($$(nproc)-1)) --filter $${SWIFT_OPENAPI_COMPATIBILITY_TEST_FILTER}"
+    command: /bin/bash -xcl "cat /proc/cpuinfo && cat /proc/meminfo && swift test --filter $${SWIFT_OPENAPI_COMPATIBILITY_TEST_FILTER}"
     environment:  # These can be overridden when running locally.
       SWIFT_OPENAPI_COMPATIBILITY_TEST_ENABLE: "true"
       SWIFT_OPENAPI_COMPATIBILITY_TEST_SKIP_BUILD: "true"