[Toybox] tests

[enh]

i'm not sure i understand the intent behind this change:

commit 7f062f2dcfa5511139476e1aef8db74f49566432
Author: Rob Landley <rob at landley.net>
Date:   Tue Nov 20 17:50:05 2018 -0600

    Skip tests that don't have the executable bit set unless $TEST_ALL set.


i assumed this was to avoid running tests for stuff in pending, but it
seems like there's quite a random mix of tests not being run
currently.

even basics like date, test, touch, and xargs aren't having their
tests run. oversight?

either way, i wonder whether it would make more sense to have `make
tests` run _all_ the tests and add something like `make quicktests` if
that was your intention? or have a tests/pending/ directory if _that_
was your intention?

and it wouldn't hurt to modify the `make help` output too, if not
running all the tests was actually intentional:

  tests           - Run test suite against all compiled commands.
                    export TEST_HOST=1 to test host command, VERBOSE=1
                    to show diff, VERBOSE=fail to stop after first failure.

(this doesn't affect Android directly because i just run all the tests
in my runner. but it may explain how i've managed to send you a few
patches that didn't pass their tests... i only learned this today
when, out of curiosity, i did a clang coverage run to see what the
test coverage looks like and was surprised to see apparently missing
coverage for tests i'd helped write myself!)