gstreamer1.0: skip a test that is known to be flaky

The aggregator testcase test_infinite_seek_50_src_live is known upstream
to be flaky[1] and when this fails in their CI they just ignore it.

It's failing often on our autobuilder, so disable the test case for now
until upstream have resolved this issue.

[ YOCTO #15054 ]

[1] https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/410



Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit c2c9cbc1)
Signed-off-by: Steve Sakoman <steve@sakoman.com>