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>

packages/twenty-server/jest-integration.config.ts

@@ -1,5 +1,6 @@
 import { JestConfigWithTsJest, pathsToModuleNameMapper } from 'ts-jest';
 
+const isBillingEnabled = process.env.IS_BILLING_ENABLED === 'true';
 // eslint-disable-next-line @typescript-eslint/no-var-requires
 const tsConfig = require('./tsconfig.json');
 
@@ -9,7 +10,9 @@ const jestConfig: JestConfigWithTsJest = {
   moduleFileExtensions: ['js', 'json', 'ts'],
   rootDir: '.',
   testEnvironment: 'node',
-  testRegex: '.integration-spec.ts$',
+  testRegex: isBillingEnabled
+    ? 'integration-spec.ts'
+    : '^(?!.*billing).*\\.integration-spec\\.ts$',
   modulePathIgnorePatterns: ['<rootDir>/dist'],
   globalSetup: '<rootDir>/test/integration/utils/setup-test.ts',
   globalTeardown: '<rootDir>/test/integration/utils/teardown-test.ts',