lib/oeqa/selftest/bblayers: use dashed subcommands

lib/oeqa/selftest/bblayers: use dashed subcommands

bitbake-layers subcommands with underscores are the old syntax; the
dashed form has been supported (and displayed in the help text) for
quite a while now, and the old syntax is about to be unsupported, so use
the dashed form in the tests.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>