Add mention about testing module limitation on usage of HttpAdapterHost.httpAdapter

[micalevisk]

I believe we should mention about this: nestjs/nest#8076 (comment) limitation of Test.createTestingModule somewhere in the docs (peharps as a sub-section of https://docs.nestjs.com/fundamentals/testing#end-to-end-testing or at https://docs.nestjs.com/faq/http-adapter page) because the project generated by Nest CLI is using it for E2E and users may face errors like this one: notiz-dev/nestjs-prisma#74 when some 3rd-party nestjs lib uses HttpAdapter under the hood