tests: kernel: replace CONFIG_USERSPACE with CONFIG_TEST_USERSPACE

This commit replaces several CONFIG_USERSPACE=y
settings with CONFIG_TEST_USERSPACE=y. This allows
the test sub-system Kconfig structure to control
the settings of USERSPACE and HW_STACK_PROTECTION
in the various tests suites.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>