Support JUnit

[jGleitz]

JUnit would be a good addition to the supported test runners.

Acceptance criteria:

1. There is a submodule in this repository for JUnit.
2. The submodule offers a JUnit extension that can be added to a unit test using JUnit's @ExtendWith.
3. When the extension is applied, @Test method can declare a parameter of type TestFiles, which will be provided by the extension (see Parameter Resolution in the JUnit Docs)
4. The TestFiles instance behaves analogous to the integrations for kotest and spek.