CMake: tests: Support skipping unsupported test with reason

Add a new argument `TEST_SKIPPED` to `mbed_greentea_add_test()` to
indicate a test is skip and give a reason (e.g. the test does not
support the target and/or configuration used).

This is achieved with a custom target in place of an executable
while keeping the same target name. The skipping message is printed
at both configuration and build stages.

tools/cmake/mbed_greentea.cmake

@@ -14,8 +14,12 @@ set(MBED_ROOT ${CMAKE_CURRENT_LIST_DIR}/../.. CACHE INTERNAL "")
 # TEST_INCLUDE_DIRS - Test suite include directories for the test
 # TEST_SOURCES - Test suite sources
 # TEST_REQUIRED_LIBS - Test suite required libraries
+# TEST_SKIPPED - Reason if suite is skipped
 #
 # calling the macro:
+# if(MBED_TEST_BAREMETAL)
+#     set(skip_reason "RTOS required")
+# endif()
 # mbed_greentea_add_test(
 #     TEST_NAME
 #         mbed-platform-system-reset
@@ -27,11 +31,16 @@ set(MBED_ROOT ${CMAKE_CURRENT_LIST_DIR}/../.. CACHE INTERNAL "")
 #     TEST_REQUIRED_LIBS
 #         mbed-kvstore
 #         mbed-xyz
+#     TEST_SKIPPED
+#         ${skip_reason}
 # )
 
 macro(mbed_greentea_add_test)
     set(options)
-    set(singleValueArgs TEST_NAME)
+    set(singleValueArgs
+        TEST_NAME
+        TEST_SKIPPED
+    )
     set(multipleValueArgs
         TEST_INCLUDE_DIRS
         TEST_SOURCES
@@ -43,42 +52,53 @@ macro(mbed_greentea_add_test)
         "${multipleValueArgs}"
         ${ARGN}
     )
-    add_subdirectory(${MBED_ROOT} build)
+    if(NOT "${MBED_GREENTEA_TEST_SKIPPED}" STREQUAL "")
+        set(msg "Skipping ${MBED_GREENTEA_TEST_NAME}: ${MBED_GREENTEA_TEST_SKIPPED}")
+        message(DEBUG "${msg}")
+        add_custom_target(${MBED_GREENTEA_TEST_NAME}
+            ALL
+            COMMAND ${CMAKE_COMMAND} -E echo ${msg}
+        )
+    else()
 
-    add_executable(${MBED_GREENTEA_TEST_NAME})
+        add_subdirectory(${MBED_ROOT} build)
 
-    # Explicitly enable BUILD_TESTING until CTest is added to the Greentea client
-    set(BUILD_TESTING ON)
+        add_executable(${MBED_GREENTEA_TEST_NAME})
 
-    target_include_directories(${MBED_GREENTEA_TEST_NAME}
-        PRIVATE
-            .
-            ${MBED_GREENTEA_TEST_INCLUDE_DIRS}
-    )
+        # Explicitly enable BUILD_TESTING until CTest is added to the Greentea client
+        set(BUILD_TESTING ON)
 
-    target_sources(${MBED_GREENTEA_TEST_NAME}
-        PRIVATE
-            ${MBED_GREENTEA_TEST_SOURCES}
-    )
+        target_include_directories(${MBED_GREENTEA_TEST_NAME}
+            PRIVATE
+                .
+                ${MBED_GREENTEA_TEST_INCLUDE_DIRS}
+        )
 
-    if(MBED_TEST_BAREMETAL)
-        list(APPEND MBED_GREENTEA_TEST_REQUIRED_LIBS mbed-baremetal)
-    else()
-        list(APPEND MBED_GREENTEA_TEST_REQUIRED_LIBS mbed-os)
-    endif()
+        target_sources(${MBED_GREENTEA_TEST_NAME}
+            PRIVATE
+                ${MBED_GREENTEA_TEST_SOURCES}
+        )
 
-    list(APPEND MBED_GREENTEA_TEST_REQUIRED_LIBS greentea::client_userio mbed-greentea-io mbed-unity mbed-utest)
+        if(MBED_TEST_BAREMETAL)
+            list(APPEND MBED_GREENTEA_TEST_REQUIRED_LIBS mbed-baremetal)
+        else()
+            list(APPEND MBED_GREENTEA_TEST_REQUIRED_LIBS mbed-os)
+        endif()
 
-    target_link_libraries(${MBED_GREENTEA_TEST_NAME}
-        PRIVATE
-            ${MBED_GREENTEA_TEST_REQUIRED_LIBS}
-    )
+        list(APPEND MBED_GREENTEA_TEST_REQUIRED_LIBS greentea::client_userio mbed-greentea-io mbed-unity mbed-utest)
+
+        target_link_libraries(${MBED_GREENTEA_TEST_NAME}
+            PRIVATE
+                ${MBED_GREENTEA_TEST_REQUIRED_LIBS}
+        )
+
+        mbed_set_post_build(${MBED_GREENTEA_TEST_NAME})
 
-    mbed_set_post_build(${MBED_GREENTEA_TEST_NAME})
+        option(VERBOSE_BUILD "Have a verbose build process")
+        if(VERBOSE_BUILD)
+            set(CMAKE_VERBOSE_MAKEFILE ON)
+        endif()
 
-    option(VERBOSE_BUILD "Have a verbose build process")
-    if(VERBOSE_BUILD)
-        set(CMAKE_VERBOSE_MAKEFILE ON)
-    endif()
+    endif() # NOT MBED_GREENTEA_TEST_SKIPPED
 
 endmacro()