tests: net: socket: tcp: Disable preempt test for mps2_an385

This commit disables the `net.socket.tcp.preempt` test on the
`mps2_an385` platform because it fails very frequently due to the
system timer stability issues.

For more details, refer to the issue zephyrproject-rtos/zephyr#48608.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>