Add Integration and unit tests on Billing (#9317)

Solves [ https://github.com/twentyhq/private-issues/issues/214 ]

**TLDR**
Add unit and integration tests to Billing. First approach to run jest
integration tests directly from VSCode.

**In order to run the unit tests:**
Run unit test using the CLI or with the jest extension directly from
VSCode.

**In order to run the integration tests:**
Ensure that your database has the billingTables. If that's not the case,
migrate the database with IS_BILLING_ENABLED set to true:
` npx nx run twenty-server:test:integration
test/integration/billing/suites/billing-controller.integration-spec.ts`

**Doing:**
- Unit test on transformSubscriptionEventToSubscriptionItem
- More tests cases in billingController integration tests.

---------

Co-authored-by: Félix Malfait <felix.malfait@gmail.com>
Co-authored-by: Weiko <corentin@twenty.com>
Co-authored-by: Charles Bochet <charlesBochet@users.noreply.github.com>

.vscode/settings.json

@@ -49,4 +49,6 @@
   "files.associations": {
     ".cursorrules": "markdown"
   },
+  "jestrunner.codeLensSelector": "**/*.{test,spec,integration-spec}.{js,jsx,ts,tsx}"
+  }
 }