dtc: More detailed testing of tree checks

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch modifies the dtc-checkfails.sh testcase wrapper so that
> instead of testing just that dtc fails with a particular error code on
> the sample input, it scans dtc's stderr output looking for a message
> that dtc failed a specific check or checks.  This has several advantages:
> 	- It means we more precisely check dtc's checking behaviour
> 	- It means we can check for generation of warnings using the
> same script
> 	- It means we can test cases where dtc should generate
> multiple errors or warnings from different checks
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl