Follow-up to #466.

Many ext tests use the default SDK TracerProvider and MeterProvider types. Some ext packages (typically "integrations") only rely on the API package, other ext packages (typically "exporters") rely on the SDK package because they use types defined in the API.

Both kinds of ext packages should work with the default no-op providers, built-in SDK providers, and custom SDK providers. Even though the SDK package needs to be present to use an exporter ext package, the user may configure their application with DefaultProvider from the API package. For this reason, the exporter shouldn't rely on SDK-specific implementation details.

To make sure we're not relying on the built-in SDK implementation, and to support writing custom SDKs, we should refactor ext tests to use each available provider. In general, custom SDKs should behave the same way as the built-in SDK, but we will want separate tests for API-provided providers.