net-p2p/syncthing: work around ppc64 test failure

net-p2p/syncthing: work around ppc64 test failure

For some reason one of the Syncthing tests - TestIssue5063 - times out
on our ppc64 buildhost, with 100% reproducibility. Upstream were not
aware of the problem because apparently they have never run the test
suite on such a system, that said their suggestion to adjust the
time-out threshold appears to do the trick. Make it 20 s i.e. twice what
it originally is, hopefully this will prevent the problem from
resurfacing any time soon.

Closes: https://bugs.gentoo.org/717108
See-also: https://github.com/syncthing/syncthing/issues/6539
Signed-off-by: Marek Szuba <marecki@gentoo.org>