Support vendoring this test suite by removing need for `array-api` submodule

[honno]

In #104 we introduce a git submodule of the array-api repo, which going forward is required for the test suite to function (e.g. special case tests). This is not ideal for some vendoring use cases where array libraries such as NumPy might like to ship the test suite in their own repo, and a git submodule would be a big nuisance to that end.

There are two current options I see:

1. Hard code the signatures files ala the ones we generated with generate_stubs.py. A pre-commit hook could alleviate the pain somewhat of having an out-of-sync repo, which was a common problem before.
2. Allow the user to specify somewhere (variable? env? package?) where the signatures folder/package is, so someone vendoring can vendor the spec too and use that.

[honno]

Relevant https://github.com/data-apis/array-api/issues/488