[java attacher] add automated tests

[stuartnelson3]

Recent releases of apm-server come bundled with the java-attacher.jar, which the apm-server can be configured to launch as a subprocess.

Create automated tests to verify that this works on the various operating systems we target.

[simitt]

~This requires elastic/observability-robots#1036 to be in place.~

not OS specific anymore

[simitt]

The overall attacher logic needs to be covered by automated tests. Please take a look and close if already covered by the latest refactoring work from @eyalkoren, or add tests accordingly.