ztest: Remove zexpect API test unnecessary timeout

The zexpect API tests included a 15 sec timeout. This is an unnecessarily
short timeout and not consistent with the rest of the ztest API tests,
which don't include timeouts themselves. Particularly this timeout seems to
go off on slower hardware.

Remove unnecessary timeout. Fixes
https://github.com/zephyrproject-rtos/zephyr/issues/55335

.

Signed-off-by: Aaron Massey <aaronmassey@google.com>