Add using direcitve to define tests within directory

[romanowski]

Currently, scala-cli recognizes only recognizes files as tests if:

• file name ends up with .test.scala
• file comes from a in directory input and it is a member (may be transitively) of test subdirecory
• file contains using target test directive

In most build tools (sbt, Mill or Gradle) there is an option to define a test directory. Maybe we can introduce test.sources directive that will list a directories (or even patters) of files / directories to be treated as tests, relative to the current directory.