ztest: include: unittest: relocate cpu.h and include only if unit-test

The `cpu.h` is meant to be a hack around Zephyr's dependencies
for unit-test, relocate it to a folder that's included only by
unit-test.

Signed-off-by: Yong Cong Sin <[email protected]>

Author: ycsin

cmake/modules/unittest.cmake

@@ -81,6 +81,7 @@ endforeach()
 
 list(APPEND INCLUDE
   subsys/testsuite/ztest/include/zephyr
+  subsys/testsuite/ztest/unittest/include
   subsys/testsuite/include/zephyr
   subsys/testsuite/ztest/include
   subsys/testsuite/include