test: redirect STDIN from /dev/null

test: redirect STDIN from /dev/null

Without this stdin may be anything that parent process provided for it.

Test processes might have tried to read something from it, which would
have caused undeterministic behavior.

E.g. gdb(1) tries to do tty related ioctls on fd 0 (and fd 1 and fd 2,
but those are redirected to 'test.output' before test runs).