Recently a classpath misconfiguration appeared in `engine/runtime` project:

![org.junit not found](https://github.com/enso-org/enso/assets/26887752/215a074b-1fad-4d4d-a5a8-a573b0d53e76)

The problem was caused by the `XyzTest.java` files being incorrectly assigned to `engine/runtime/src/main/java` source root and its classpath (which obviously doesn't contain JUnit). This PR restricts the `srcCp` to `inputDir`, when it is known. That properly assignes the `XyzTest.java` files to `engine/runtime/src/test/java` source root. The IGV as well as VSCode support seems to recognize unit test classpath properly now.