Test auto completion when typing values

[janpio]

Example: When you have @relation(onUpdate: C| it should suggest auto completion Cascade.

It was working before merging https://github.com/prisma/language-tools/pull/847

We do not have any tests for that yet.

Note: Probably only makes sense as follow up of https://github.com/prisma/language-tools/issues/849 as this could be implemented as an automation of the test system (test if auto completeion shows up if you type first character of item that is supposed to show up)

[janpio]

Besides the @relation auto completion, this could also be affect the auto complete of the preview feature flags.