ztest: add support for new and legacy include paths

The ztest unittest module relies on some headers that fake the real
ones, e.g. <zephyr/arch/cpu.h>. When new header location was introduced,
their path was not updates. This patch fixes this problem and provides
ztest with both, legacy and new include paths so that it can keep
working on both scenarios.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>