Update gtest_helper.cmake

This updates the add_icon_c_test macro in cmake/gtest_helper.cmake:

1. The macro is turned into a function to avoid namespace pollution.
2. The function is renamed to fs_add_c_test.
3. The function now accepts named options SOURCES (followed by a list of source files of the test) and ARGS (followed by a list of command-line arguments for the test).
4. The old signature of the macro is still supported, i.e. it is possible to call something like add_c_test(UtilHashTest ctest_util_hash.cpp), but it made sense to me to switch to the new one.
5. The UtilFileTest test is now provided with an extra command-line argument and it runs on ${CMAKE_CURRENT_BINARY_DIR} instead of . as it was before !28 (merged).