It's really annoying to add a message after each check in a unit test.
Let's make this optional. If the message is omitted, "line %d" will be
used instead. Also, let's print the expression on failure because it
may be useful if exact sources are unavailable.

NO_DOC=refactoring
NO_CHANGELOG=refactoring

There's no need to duplicate all unit test helpers for TAP compatible
tests. The only difference between them is that plan() prints the TAP
version so let's do just that.

NO_DOC=code cleanup
NO_CHANGELOG=code cleanup

Unit test coio_stat.

Cleanup unit tests a bit: merge unit.h and test.h, which
would duplicate each other.

Fix a bug in trigger_atexit.test, which would not clean up after
itself.