cmake: set IS_TEST in the hello_world sample

The Kbuild equivalent of IS_TEST was used for hello_world before, so
we revert back to the old behaviour. This resolves
https://github.com/zephyrproject-rtos/zephyr/issues/4904

Signed-off-by: Sebastian Boe <sebastian.boe@nordicsemi.no>
diff --git a/samples/hello_world/CMakeLists.txt b/samples/hello_world/CMakeLists.txt
index 6daa0b7..2a129c2 100644
--- a/samples/hello_world/CMakeLists.txt
+++ b/samples/hello_world/CMakeLists.txt
@@ -1,3 +1,5 @@
+set(IS_TEST 1)
+
 include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
 project(NONE)