Move String.uppercase to a single place

Don't use String.uppercase_ascii yet, since it's not available in
Debian stable's OCaml package.
Also get rid of some leading underscores.

* src/util.ml (string_uppercase): New function.
* tests/utest_lib.ml (must_run_section, must_run_title):
* tests/utest_main.ml (must_run): Use it and simplify.