Rename "drop-in test" (DropIn) to "simple test" (SimpleTest).

Due the concept of "drop-in" tests didn't match the actual
purpose of running any script or binary as a test in Avocado, we're
going to use the term "simple test" (defined in SimpleTest class)
to contrast with "native test", which are Python modules that
uses Avocado's API.

Signed-off-by: Rudá Moura <rmoura@redhat.com>