tests: test-filter: Avoid gcc 14 warning about uninitialized variable
This is kinda of a false positive as the varialbe is never accessed uninitialized. Initialize it to silence the wanring.
../subprojects/gstreamer-vaapi/tests/internal/test-filter.c: In function ‘main’:
../subprojects/gstreamer-vaapi/tests/internal/test-filter.c:389:10: error: ‘denoise_level’ may be used uninitialized [-Werror=maybe-uninitialized]
389 | if (!gst_vaapi_filter_set_denoising_level (filter, denoise_level))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../subprojects/gstreamer-vaapi/tests/internal/test-filter.c:333:11: note: ‘denoise_level’ was declared here
333 | gdouble denoise_level, sharpen_level;
| ^~~~~~~~~~~~~