fix(healthcare): remove mocha parallel execution to prevent npm install race conditions in CI

Author: angelcaamal

healthcare/fhir/package.json

@@ -9,7 +9,7 @@
     "node": ">=12.0.0"
   },
   "scripts": {
-    "test": "c8 mocha -p -j 2 system-test/*.test.js --timeout=60000"
+    "test": "c8 mocha system-test/*.test.js --timeout=60000"
   },
   "devDependencies": {
     "@google-cloud/pubsub": "^4.0.0",