selftest/recipetool: Split tests into separate test classes

Split tests into separate test classes to speed up individual test runs
by reducing the test setup to a minimum.

The pkgdata generation is only needed for the append tests and slow down
the other tests.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>