tests:nle: Unref the bus before unrefing the pipeline

tests:nle: Unref the bus before unrefing the pipeline

Aiming at fixing a rare race condition where we get:

    ../subprojects/gstreamer/libs/gst/check/gstcheck.c:1258:F:nlecomposition:test_seek_on_nested:0: nested_src0_0x1a1a310 is not destroyed, 1 refcounts left!

The idea is that there might have a remaining GstMessage
with the nested_src as `message.src` on the bus that has
yet to be processed in some conditions leading to a reference
still existing when unrefing the pipeline.