"addtask do_XXX before YYY after ZZZ "
where YYY or ZZZ is missing the "do_" prefix don't work as expected. Ajust the
code so that it doesn't just silently do the wrong thing but works as expected.
Expand a test case to cover this.
(Bitbake rev: 21670b9bb8936ec44aedff26163948bbc2ceb44a)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>