test: Merge _should_test_azure and _get_azure_url inside Azure test helper

[skshetry]

I tried doing this in a single PR, but, it became fairly large. So, I'm just making a PR with a single change.

After merging all the functions inside tests/remotes.py, we could use the {Azure, S3, GCP, etc.} classes as mixins on the TestCases.

Related to #2878.

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[ghost]

The logic is different from other should_test, for example:

class GDrive:
    @staticmethod
    def should_test():
        return os.getenv("GDRIVE_USER_CREDENTIALS_DATA") is not None

At least as credentials are set, it will run the tests.

But as I'm reading this, if DVC_TEST_AZURE is defined, it is assumed that AZURE_STORAGE_{CONTAINER_NAME,CONNECTION_STRING} are set.

Why not going for testing only the credentials?

[efiop]

@MrOutis Gdrive is the only one that doesn't respect the previous convention 🙂 This change does. DVC_TEST* are useful when you want to ensure that your tests will run even if the env vars are not defined, for example when you have some other way of providing credentials already. For example, should_test for s3 tests for AWS_ env vars, but I don't define them locally, since I have the default ones that work as is.

[ghost]

@efiop , got it 👍

[ghost]

@efiop , got it 👍