test: remove asset test that needs internal method

test: remove asset test that needs internal method

The test_proxy_asset test needs the internal method
ges_asset_finish_proxy. The test also uses the associated internal methods
ges_asset_try_proxy and ges_asset_cache_lookup. However, these are
marked with GES_API in ges-internal.h, which allows us access to them
here.
The new method is not marked as GES_API because it would not allow us to
remove the method in the future without removing it from the symbols list.
We do not want to add to the problem.

The test was simply commented out since we may wish to support tests
that access internal methods in the future using meson.