Add support to filter test configurations depending on the subscription level of the running stack

The elastic-package has been updated to allow users to start the Elastic stack with different subscription levels (e.g. basic) since the pull request #2501. By default, the subscription used in the Elastic stacks managed by `elastic-package` is `trial`.

After adding new builds to test packages with the `basic` subscription in the integrations repository, there is a package failure due to a processor not available with that subscription level (more info at https://github.com/elastic/integrations/issues/13503)

To address this, we propose adding a new setting in the test configuration files to specify the required subscription level. This setting could be added as follows:

```diff
@@ -1,3 +1,4 @@
+subscription: platinum
multiline:
first_line_pattern: "^\\d{4}\\/\\d{2}\\/\\d{2} "
fields:
"@timestamp": "2020-04-28T11:07:58.223Z"
dynamic_fields:
"event.ingested": ".*"
```

Scenarios in test configuration:
- No subscription key: All test configurations without subscription setting are tested.
- `subscription: basic`: Test configurations with this subscription level will always be tested.
- `subscription: platinum|enterprise`: Test configurations with this subscription levels will only be tested if the stack subscription is `trial`.

This new setting in test configuration should be available for both pipeline and system tests.

Start by tracing how elastic-package loads test configuration files for pipeline and system tests, then follow the existing stack subscription handling introduced by pull request #2501. Done means configurations without a subscription remain included, basic configurations are always tested, and platinum or enterprise configurations run only with a trial stack.

Written by the indexing model from the issue text.