build: add code coverage in GitHub (#1894)

Author: harshachinta

package.json

@@ -33,7 +33,7 @@
  "samples-test": "cd samples/ && npm link ../ && npm test && cd ../",
  "system-test": "mocha build/system-test --timeout 1600000",
  "cleanup": "mocha scripts/cleanup.js --timeout 30000",
- "test": "c8 mocha build/test build/test/common",
+ "test": "mocha build/test build/test/common",
  "ycsb": "node ./benchmark/ycsb.js run -P ./benchmark/workloada -p table=usertable -p cloudspanner.instance=ycsb-instance -p operationcount=100 -p cloudspanner.database=ycsb",
  "fix": "gts fix",
  "clean": "gts clean",
@@ -47,7 +47,8 @@
  "benchwrapper": "node bin/benchwrapper.js",
  "prelint": "cd samples; npm link ../; npm install",
  "precompile": "gts clean",
- "compile-protos": "compileProtos src"
+ "compile-protos": "compileProtos src",
+ "coverage": "c8 mocha build/test build/test/common && c8 report --check-coverage"
  },
  "dependencies": {
  "@google-cloud/common": "^4.0.0",