api-tests: fix CMakeLists include on non-Linux

Fixes: #246

Credits to @ccli8(ccli8@nuvoton.com) for finding the fix.

When we use CMake's CMAKE_MODULE_PATH feature, the module name
being included should not contain slashes ("/") which can't be
resolved by CMake on Windows and macOS hosts, resulting in
failure of `include("common/Utils")` on those systems.

Since other modules are included with full paths which work in
all cases, do the same for common/Utils.cmake.
diff --git a/api-tests/CMakeLists.txt b/api-tests/CMakeLists.txt
index 98ae325..306ce6f 100644
--- a/api-tests/CMakeLists.txt
+++ b/api-tests/CMakeLists.txt
@@ -28,8 +28,7 @@
 
 get_filename_component(PSA_ROOT_DIR . ABSOLUTE)
 
-list(APPEND CMAKE_MODULE_PATH ${PSA_ROOT_DIR}/tools/cmake)
-include("common/Utils")
+include(${PSA_ROOT_DIR}/tools/cmake/common/Utils.cmake)
 include(${PSA_ROOT_DIR}/tools/cmake/common/CMakeSettings.cmake)
 include(${PSA_ROOT_DIR}/tools/cmake/common/CMakeExternal.cmake)