tests: refactor tests and add n-test-util helper library

- we have "check-all.c" and "check-direct.c", which contains the
  main functions of the actual tests.
  On the other hand, the other "check-{addr,attr,ematch-tree-clone}.c"
  files only contained the test suites for "check-all.c". Rename
  the latter to have a separate name prefix.

- rename "tests/util.h" to "tests/cksuite-all.h". It's really the
  header that declares all the suites.

- add a "tests/nl-test-util.c" as a static helper library with
  test code.