test: add test for getMediaKeySystemAccess();

[Florent-Bouisset]

add unit test for getMediaKeySystemAccess()

[peaBerberian]

I think tests for getMediaKeySystemAccess should not be added to __global__/ (but directly inside __tests__/) because __global__ tests basically are integration tests for the ContentDecryptor as a whole, not its inner functions.

The idea of __global__/ was to have a middle ground between:

• our unit tests, which tested individual functions (not exactly what you have here, but it is still close to root utils)

  The cons for those were that they very often needed to be updated because e.g. the decrypt/ inner logic at the function-level changes very often and they also empirically have been much less useful than integration tests at discovering issues in the RxPlayer: they usually only broke when the thing they do is no more aligned with the code they test.

• our full-RxPlayer integration tests, which are much more stable (as they just follow our API), but for which specific edge states might be difficult to achieve

So we decided some years ago to have test relying on the module's (in this case the ContentDecryptor) API.
Those API are not part of our semver as they are not exposed to applications, yet because those modules interact with other unrelated modules inside the RxPlayer, they still follow generally a much more stable behavior than those inner functions.
And that's what __global__/ was for: in some sense those are integration tests for the module.

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Moreover just to clarify here, I've nothing against unit tests at lower levels, and I think that it's a net good thing to add some, most of all for testing that some code does what we think it does in some edge cases.
But historically we've put much less effort on them when compared with tests testing things at higher abstraction levels because the latter historically have been both more helpful and stable.

[peaBerberian]

⚠️ we cannot in many cases just set the asked configuration as the return value of getConfiguration

e.g. distinctiveIdentifier is returned as optional, something that it cannot be:

All values in the configuration may be used in any combination. Members of type MediaKeysRequirement reflect whether the capability is required for any combination. They will not have the value "optional".

Anyway we don't care for this test and this is a nitpick