[C/C++] Unittests with googletest

This MR adds C unit tests using googletest.

cmake/gtest_helper.cmake

+macro(add_icon_c_test test_name file_name)
+    add_executable("CTest_${test_name}" ${file_name})
+    target_include_directories("CTest_${test_name}" SYSTEM AFTER PUBLIC "${CMAKE_SOURCE_DIR}/test/googletest/include" ${CMAKE_CURRENT_BINARY_DIR})
+    target_link_libraries("CTest_${test_name}" PRIVATE fortran-support gtest gtest_main)
+    add_test(NAME "CTest_${test_name}" COMMAND "CTest_${test_name}")
+    set_property(TEST "CTest_${test_name}"
+        PROPERTY LABELS C)
+endmacro()
+
+macro(add_icon_fortran_test test_name file_name)
+    add_executable("FTest_${test_name}" ${file_name})
+    target_include_directories("FTest_${test_name}" SYSTEM AFTER PUBLIC "${CMAKE_SOURCE_DIR}/test/googletest/include")
+    target_link_libraries("FTest_${test_name}" PRIVATE FTest_C_Binding fortran-support gtest gtest_main)
+    add_test(NAME "FTest_${test_name}" COMMAND "FTest_${test_name}")
+    set_property(TEST "FTest_${test_name}"
+        PROPERTY LABELS Fortran)
+endmacro()
+
+macro(add_icon_fortran_c_test test_name file_name)
+    add_executable("FCTest_${test_name}" ${file_name})
+    target_include_directories("FCTest_${test_name}" SYSTEM AFTER PUBLIC "${CMAKE_SOURCE_DIR}/test/googletest/include")
+    target_link_libraries("FCTest_${test_name}" PRIVATE FTest_C_Binding fortran-support gtest gtest_main)
+    add_test(NAME "FCTest_${test_name}" COMMAND "FCTest_${test_name}")
+    set_property(TEST "FCTest_${test_name}"
+        PROPERTY LABELS Fortran/C)
+endmacro()