Bluetooth: tester: Fix bt-stack-tester to unix domain socket

/tmp/bt-stack-tester is not a pipe, but unix domain socket.
This commit fixes respective "make run" errors:

qemu-system-arm: -serial pipe:/tmp/bt-stack-tester: Could
not open '/tmp/bt-stack-tester': No such device or address

qemu-system-arm: -serial pipe:/tmp/bt-stack-tester: could
not connect serial device to character backend
'pipe:/tmp/bt-stack-tester

Signed-off-by: Ruslan Mstoi <ruslan.mstoi@intel.com>