tests: add missing return
tests: add tests to test-util

add tests for:
  - filename_is_safe
  - ascii_strlower
  - files_same
  - is_valid_documentation_url
  - file_in_same_dir
  - endswith
  - close_nointr
  - unlink_noerrno
  - readlink_and_make_absolute
  - read_one_char
  - ignore_signals
  - strshorten
tests: add tests for strv_extendf
util.c: simplify rm_rf_children_dangerous