chore: add an npm test:once script

[lance]

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

> [email protected] 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

> [email protected] 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 [email protected]