test-json: Avoid use of /dev/stdin to fix builds in limited chroots.

test-json: Avoid use of /dev/stdin to fix builds in limited chroots.

The chroots in which we often build Open vSwitch don't have /proc and
thus cannot support /dev/stdin, because on Linux that is a symlink to
/proc/self/fd/0.  So avoid using /dev/stdin in the testsuite.