[tests] gc-descriptors: Only copy file in case of VPATH build

Instead of ignoring the cp error when copying over the same file in non-VPATH builds.

This is purely a convenience fix for one of my scripts that processes the log, without this the log shows the following:
> cp: ‘descriptor-tests-driver.cs’ and ‘./descriptor-tests-driver.cs’ are the same file
> cp: ‘descriptor-tests-prefix.cs’ and ‘./descriptor-tests-prefix.cs’ are the same file
> cp: ‘gen-descriptor-tests.py’ and ‘./gen-descriptor-tests.py’ are the same file
> make[4]: [descriptor-tests.cs] Error 1 (ignored)