Support testing inputs that generate multiple signals

[jsoriano]

With the introduction of support for OTel collector receivers in inputs, there can be single inputs that collect multiple signals, that go to different data streams. This can be dynamic, depending on the configuration, when dynamic signals are enabled.

So far elastic-package has been expecting documents on a single input, so it cannot test scenarios like these ones with multiple signals.

There are two strategies that could be followed to cover these cases:

• elastic-package uses Fleet/Elasticsearch APIs to discover the data streams created for a given policy.
• Developers define in tests on what signals they expect to find documents.

Both would be complementary, the first one would be the default, the second one could be used in test cases where the developer explicitly wants to test that a given configuration collects an specific type of signal.

If elastic-package becomes able to detect the data streams created for a given policy, we could remove the logic we have to build the data stream names where it is expected to find data. It should be also able to keep working without relying on the input type configured in the policy (elastic/package-spec#1094).