Add internal utility function for splitting a string into string views

* src/util.cpp, src/util.h (split_into_string_views): New.
* tests/CMakeLists.txt: Add macro _g10_add_test. Add new test.
* tests/t-utils.cpp: New.

The internal function is exported to make it available in the test.

• GnuPG-bug-id: T7883