passtest: introduce simplest possible test that passes

The idea is twofold:

1) Show that a test passes simply by not failing, and doing nothing
   means it can't fail
2) Where appropriate, use this in our own functional tests to reduce
   the selftest runtime.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
selftests: replace sleeptest with passtest

This patch replaces most of the calls to sleeptest in our functional
test with passtest, in an attempt to reduce, without invalidating the
purporse, the time it takes to run them.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #267 from clebergnu/example_passtest

Introduce example test: passtest