fix Java Testrunner

[fbergmann]

Description

This commit changes the auto test runner to include tests from sub directories, rather than just test. While previous javac versions compiled the old version without issue, newer javac compilers raise an issue. it was originally submitted by @pgrt

Motivation and Context

The change is needed, to get the java bindings built with newer javac, as it otherwise will fail when compiling the tests.
the change still works for older javac versions as well.

fixes #281

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

this will be tested on a run of the extensive checks.

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically