chore: specify MongoDB conn string in action:coverage

This makes sure we explicitly specify the MongoDB connection
string in `action:coverage` to address an issue I observed in
CI for the action would sometimes fail to connect.

It also modifies the `action:coverage` command to delegate to
the `test` command, so that any future updates to how tests
are run is automatically picked up by the coverage checker.

Signed-off-by: Andrew Lilley Brinker <abrinker@mitre.org>

Author: alilleybrinker

package.json

@@ -77,7 +77,7 @@
     },
     "scripts": {
         "action:test": "node node_modules/mocha/bin/mocha test/unit-tests --recursive --exit",
-        "action:coverage": "NODE_ENV=test node node_modules/nyc/bin/nyc.js --lines=80 node_modules/mocha/bin/mocha.js src/* --recursive --exit",
+        "action:coverage": "NODE_ENV=test MONGO_CONN_STRING=mongodb://localhost:27017 MONGO_DB_NAME=cve_test node node_modules/nyc/bin/nyc.js --lines=80 npm run test",
         "action:lint-src": "node node_modules/eslint/bin/eslint.js src/",
         "action:lint-test": "node node_modules/eslint/bin/eslint.js test/",
         "action:lint-test-utils": "node node_modules/eslint/bin/eslint.js test-utils/",