dt-parser: relax parsing of dt_validate_schema

The warning messages formats have changed a bit with recent dt-schema tools
and the dt-parser is not able to determine the schema corresponding to a
warning anymore.
Relax the parsing a bit to repair the dt_validate_schema properties.

While at it, add a unit test.

[Backport commit 6297fabf.]
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>