oeqa/runtime_test: use subtests in test_postinst_rootfs_and_boot

oeqa/runtime_test: use subtests in test_postinst_rootfs_and_boot

As this test has two nested loops and actually runs six times, use
UnitTest.subTest() so we can tell which instance is failing, and to run all
variations instead of failing on the first one.

Also set PACKAGE_CLASSES to just the type we need to reduce the verboseness of
the output, and consolidate the feature generation to be neater.

(From OE-Core rev: c7ef6000b11f1b1cd27c9bc408eea9f76bb94a3b)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>