build: Parallelize 'run-tests'

Because nextest does not support running doctests, we are parallelizing
the run-tests.sh run in buildkite to run nextest in one job and doctests
in another.  This means spending a little more on CPU, but shortening
the total wall clock duration of a build (plus gaining the features
provided by nextest).

Change-Id: I8211a10ba41179d2b0ed1d888588863011df5afc

.buildkite/pipeline.public-common.yml

@@ -78,6 +78,7 @@ steps:
 
   - label: ':rust: Run tests'
     key: rust-tests
+    parallelism: 2
     commands:
       - '[ -d public ] && cd public'
       - '.buildkite/run-tests.sh'

.buildkite/run-tests.sh

@@ -11,7 +11,24 @@ upload_artifacts() {
 echo "+++ :rust: Run tests"
 export DISABLE_TELEMETRY=true
 export PROPTEST_MAX_SHRINK_TIME=1800000
-cargo --locked nextest run --workspace --features failure_injection --exclude readyset-clustertest \
-    --exclude benchmarks --ignore-default-filter \
-    && cargo --locked test --workspace --features failure_injection --exclude readyset-clustertest \
-    --exclude benchmarks --doc || upload_artifacts
+
+# If we aren't actually running run-tests.sh as a parallel job, just follow the
+# "job 0" path.
+: "${BUILDKITE_PARALLEL_JOB:=0}"
+
+if [[ "$BUILDKITE_PARALLEL_JOB" == "0" ]]; then
+    # Run nextest
+    cargo --locked nextest run --workspace --features failure_injection \
+        --exclude readyset-clustertest \
+        --exclude benchmarks --ignore-default-filter \
+        || upload_artifacts
+elif [[ "$BUILDKITE_PARALLEL_JOB" == "1" ]]; then
+    # Run doctests, because at this time nextest does not support doctests
+    cargo --locked test --workspace --features failure_injection \
+        --exclude readyset-clustertest \
+        --exclude benchmarks --doc \
+        || upload_artifacts
+else
+    echo "No command defined for this parallel job."
+    exit 1
+fi