lib: os: assert: Add unreachable path hint for assertion failure

This commit adds the `CODE_UNREACHABLE` hint at the end of the
assertion failure branch so that the compiler takes note of the assert
function not returning when an assertion fails.

This prevents the compiler from generating misguided warnings assuming
the asserted execution paths.

It also introduces the `ASSERT_TEST` Kconfig symbol, which indicates
that the "assert test mode" is enabled. This symbol may be selected by
the tests that require the assert post action function to return
without aborting so that the test can proceed.

Note that the `CODE_UNREACHABLE` hint is specified only when the assert
test mode is disabled in order to prevent the tests from crashing when
the assert post action function returns.

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