tests: drivers: uart: uart_async_api: disable userspace permissions

Test cases on F746ZG work with USERSPACE config not enabled.
PR #80944 forces the use of the TEST_ENABLE_USERSPACE config with
the "select" statement instead of "imply," which is more restrictive
for test case dependencies.This leads to the error
"Memory region 0x2004d180 (size 8) write access denied" on boards.
By setting CONFIG_TEST_USERSPACE=n, the TEST_ENABLE_USERSPACE config will
not be enabled.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>