raisin: introduce tests

raisin: introduce tests

Introduce a new command to run functional tests and unit tests.
Introduce a generic infrastrucutre to run tests on the local machine.
Add a library of common functions that can be used by the test scripts
to setup guest VMs.

Add a simple test script that boots a single busybox based PV guest.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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Changes in v3:

- source test scripts
- expose a _test and a _cleanup function from the test script

Changes in v2:

- use found as a boolean
- print error to stderr