rt-tests: switch site from Debian snapshot to Linux's git

Origin of "rt-tests" is:
git://git.kernel.org/pub/scm/linux/kernel/git/clrkwllms/rt-tests.git

Switching to this new "origin" simplifies version bumping because there's
no need in updating Debian snapshot folder as it was done already here:
http://git.buildroot.net/buildroot/commit/package/rt-tests?id=da330e508c2d95e898ac52a2aa39426a5f6d0506



Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>