tests: make find_package(Zephyr...) REQUIRED

... because it is (required).

This makes a difference when building with CMake and forgetting
ZEPHYR_BASE or not registering Zephyr in the CMake package registry.

In this particular case, REQUIRED turns this harmless looking log
statement:

-- Could NOT find Zephyr (missing: Zephyr_DIR)
-- The C compiler identification is GNU 9.3.0
-- The CXX compiler identification is GNU 9.3.0
-- Check for working C compiler: /usr/bin/cc
-- ...
-- ...
-- ...
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at CMakeLists.txt:8 (target_sources):
  Cannot specify sources for target "app" which is not built by
  this project.

... into this louder, clearer, faster and (last but not least) final
error:

CMake Error at CMakeLists.txt:5 (find_package):
  Could not find a package configuration file provided by "Zephyr" with
  any of the following names:

    ZephyrConfig.cmake
    zephyr-config.cmake

  Add the installation prefix of "Zephyr" to CMAKE_PREFIX_PATH or set
  "Zephyr_DIR" to a directory containing one of the above files.  If
  "Zephyr" provides a separate development package or SDK, be sure it
  has been installed.

-- Configuring incomplete, errors occurred!

Signed-off-by: Marc Herbert <marc.herbert@intel.com>