Add a unit test for the DefaultScriptService

For now we just test that the scijava.scripts.path system property is
handled as expected.
TestUtils: add method to create file in a sub-path

This is useful, e.g., for creating dummy scripts to test script
discovery.
TestUtilsTest: use matching package
TestUtilsTest: fix javadoc link
TestUtilsTest: add a test for createPath method
Add a basic unit test for ScriptFinder