eslint: catch .only() calls in db-migration tests (#1360)

Because db-migration tests use a wrapper for mocha describe() calls, extra configuration is required to catch .only() calls.

See: https://www.npmjs.com/package/eslint-plugin-no-only-tests#user-content-options

.eslintrc.json

@@ -28,7 +28,7 @@
     "no-else-return": "off",
     "no-multiple-empty-lines": "off",
     "no-nested-ternary": "off",
-    "no-only-tests/no-only-tests": "error",
+    "no-only-tests/no-only-tests": [ "error", { "block": [ "describe", "it", "describeMigration" ] } ],
     "no-restricted-syntax": "off",
     "no-underscore-dangle": "off",
     "nonblock-statement-body-position": "off",