check-sr-results: add should-contain property

Add a `should-contain' property, equivalent to the `must-contain' property
but issuing only a warning.
Use it in the configurations to relax our checks on SMBIOS.

Update the documentation and the schema accordingly, and add a unit test.

[Backport commit 2c3e376b.]
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>