Migrate IntelliJ plugin tests from JUnit 4 to Spockk

[pshevche]

Why does this matter?

Currently, IntelliJ tests heavily rely on JUnit 4 test fixtures. Those are rather cumbersome, and it would be great to use Spockk only for all specs in this repository. Let's see if we can make it happen (and maybe move tests over to the spockk-specs module).

What should developers be able to write?

Not applicable.

Acceptance criteria

• spockk-intellij-plugin tests are no longer using JUnit 4

How does Spock implement this?

Not applicable.

Additional context

1. The first increment would be to replace JUnit 4 tests with JUnit 5. This should definitely be possible and there are IntelliJ plugin projects that are using JUnit 5.
2. As the next step, explore whether JUnit 5 tests can be replaced with Spockk.
3. Finally, explore moving IntelliJ plugin tests over to spockk-specs.