chore: add an npm test:once script (#480)
chore: add an npm test:once script (#480)

Adds a convenience npm script that allows you to run only a single test
file at a time. Example usage:

```
sdk-javascript on  lance/update-deps-package-json [!?] is 📦 v5.3.2 via  v16.14.0
❯ npm run test:one

> cloudevents@5.3.2 test:one
> mocha --require ts-node/register

Error: No test files found

sdk-javascript on  lance/update-deps-package-json [!?] is 📦 v5.3.2 via  v16.14.0
❯ npm run test:one -- ./test/integration/batch_test.ts

> cloudevents@5.3.2 test:one
> mocha --require ts-node/register "./test/integration/batch_test.ts"

  A batched CloudEvent message over HTTP
    ✔ Can be created with a typed Message

  A batched CloudEvent message over Kafka
    ✔ Can be created with a typed Message

  2 passing (5ms)

sdk-javascript on  lance/update-deps-package-json [!?] is 📦 v5.3.2 via  v16.14.0
```

Signed-off-by: Lance Ball <637094cbcafd6c4c397ebad24eca1a25fa2cf312@redhat.com>
cloudevents/sdk-javascript,cloudevents/sdk-javascript