Update gtest_helper.cmake (!38)

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.

Co-authored-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Approved-by: Terry Cojean <terry.cojean@kit.edu>
Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: other