CI (Buildkite): Run the Distributed test suite with multithreading enabled (#42479)

(cherry picked from commit 72ec349)

Author: DilumAluthge

.buildkite/pipelines/main/platforms/tester_linux.yml

@@ -76,8 +76,16 @@ steps:
           # "MT" = multi-threaded
           export JULIA_NUM_THREADS=16
 
-          # We do not run the Distributed test suite when multithreading is enabled.
-          export TESTS="[\"all\", \"--skip\", \"Distributed\"]"
+          if [[ "$${BUILDKITE_PIPELINE_SLUG:?}"   == "julia-release-1-dot-6" ]]; then
+            # On Julia 1.6, we skip the Distributed test suite if multithreading is enabled.
+            export TESTS="[\"all\", \"--skip\", \"Distributed\"]"
+          elif [[ "$${BUILDKITE_PIPELINE_SLUG:?}" == "julia-release-1-dot-7" ]]; then
+            # On Julia 1.7, we skip the Distributed test suite if multithreading is enabled.
+            export TESTS="[\"all\", \"--skip\", \"Distributed\"]"
+          else
+            # On Julia 1.8 and later, we do not skip any test suites.
+            export TESTS="[\"all\"]"
+          fi
         fi
       fi