tests: remove duplication about testing of config.* aux files

tests: remove duplication about testing of config.* aux files

* tests/add-missing.test: Also check that the `AC_CANONICAL_SYSTEM'
autoconf macro causes the `config.sub' and `config.guess' scripts
to be installed by `automake --add-missing'.  Since we are at it,
fix minor buglets and cosmetic issues in the `check_' function.
* tests/hosts.test: Removed, completely subsumed by the previous
test now.
* tests/Makefile.am (TESTS): Update.