Bug 11757

At the moment, some tests rebuild "normal" recipes with different settings:
- system update: refkit-image-common
- secure boot: ovmf (and others?)

This causes extra work in the CI testing because:
- system update runs and modifies refkit-image-common
- image installer needs to rebuild the original refkit-image-common,
  which wouldn't be needed if system update hadn't modified it

Changing the test order could be used to work around that but that is merely a workaround and doesn't work in practice because of bug #11756.

All tests should be written so that they either use the original recipes unmodified, or use their own test-specific recipes. For example, system update should have its own refkit-system-update.bb which just includes refkit-image-common.bb.

We probably need some infrastructure for creating test recipes. OE-core does it via meta-selftest, but that doesn't scale to other layers.

Another aspect involves ovmf: running a virtual machine with "runqemu ovmf" potentially modifies the ovmf.qcow2 file when allowing image writes to persist. Each test which needs to do that (image installer!) should make a copy of ovmf.qcow2 first and use the copy.