tests : use CMake definitions for model/sample paths (#3406)

tests : use CMake definitions for model/sample paths (#3406)

This commit modifies the test-vad and test-vad-full tests to use CMake
definitions for the model and sample paths.

The motivation for this is that currently the tests use relative paths
which might not always be correct depending on the working directory.
With the changes in this commit the tests can be run usins ctest:
```console
$ ctest -R ^test-vad$ --test-dir build
```
Or directly (which is not currently possible without this fix):
```
./build/bin/test-vad
```

Resolves: https://github.com/ggml-org/whisper.cpp/issues/3404