refactor: test runtime extensions

[ndr-brt]

Feature Request

Currently the "junit extensions" for running EDC runtimes during integration/component/e2e tests are not much usable, e.g. there's no extension that could be used out of the box to run a single instance for all the tests (that would save a lot of time in running the tests).

Second, the EdcRuntimeExtension does 2 things in one: extension and runtime. Extracting an EmbeddedRuntime that could be used for testing would simplify the code and make everything clearer.

Which Areas Would Be Affected?

tests

Why Is the Feature Desired?

cleanup

Solution Proposal

If possible, provide a (brief!) solution proposal.