ztest: Test selection documentation.
ztest: Test selection documentation.

Explain usage of the `-test` command line argument for
posix builds.

Signed-off-by: Al Semjonovs <c3a2ecb550cad1e7d5a943ca6241377f7b43ffc4@google.com>

finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr