test(serverless-spark): run tests sequentially for CI (googleapis#2603)

Running tests in parallel consumes more GCE CPU quota, and we're
occasionally getting flakes due to exceeding the default. This still may
be the right tradeoff for speed of interactive development, but these
aren't (yet) the slowest integration tests, so running sequentially will
preserve quota without increasing CI walltime.

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🛠️ Fixes #<issue_number_goes_here>

Author: dborowitz

.ci/integration.cloudbuild.yaml

@@ -968,6 +968,7 @@ steps:
       - "SERVERLESS_SPARK_LOCATION=$_REGION"
       - "SERVERLESS_SPARK_PROJECT=$PROJECT_ID"
       - "SERVERLESS_SPARK_SERVICE_ACCOUNT=$SERVICE_ACCOUNT_EMAIL"
+      - "EXTRA_TEST_ARGS=-test.parallel=1"
     secretEnv: ["CLIENT_ID"]
     volumes:
       - name: "go"

.ci/test_with_coverage.sh

@@ -35,7 +35,7 @@ for tool_name in "${TOOL_PACKAGE_NAMES[@]}"; do
 done
 
 # Run integration test
-if ! ./"${TEST_BINARY}" -test.v -test.coverprofile="${COVERAGE_FILE}"; then
+if ! ./"${TEST_BINARY}" -test.v ${EXTRA_TEST_ARGS:-} -test.coverprofile="${COVERAGE_FILE}"; then
   echo "Error: Tests for ${DISPLAY_NAME} failed. Exiting."
   exit 1
 fi