support/testing: do not use s.b.o
Currently, the runtime tests will use the sources.buildroot.org backup
mirror, which is the default setup.
However, in some cases we do not want to use the backup site, because we
want to ensure that the download actually works.
This is the case for vendored packages, like cargo or golang packages,
for whbich we want to check that gthe download still works when we
update the rust or go versions, or when our download helpers change.
So, disable the use of the backup site in all runtime tests, and drop
the no-longer needed special cases.
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>