lisa.tests.base: Add OptionalDmesgTestBundle
lisa.tests.base: Add OptionalDmesgTestBundle

Subclass of DmesgTestBundleBase that only logs a warning if the
DmesgCollector cannot be created (e.g. if the target is not rooted).
This allows using it on base classes used by all tests in order to
collect dmesg artifact when possible, without making it mandatory.

ARM-software/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,credp/lisa