Skip unit tests that depend on LP solver if none is available
bugfix: clarify "locking_priority" [incompatible API change]

WARNING: This patch breaks API compatibility and requires changes by
users of the SchedCAT library's blocking analysis code.

The recent merge of the LP-based spin lock analysis introduced a confusion
of the field 'locking_priority' in the blocking analysis API. This patch
restores the old behavior, where "locking priority" really served to
to specify possible preemptions.

Further, the spin lock analysis changes "abused" locking_priority field
to indicate request priority in priority-ordered spin locks. This is
reverted to instead use the per-request ResourceRequirement.priority
attribute.

To avoid similar confusion in the future, the field and corresponding
APIs have been renamed to use the term "preemption_level".
Resolve test failure with expectation update

Patch 3b1068d75044f2a16eb0bf619bdb1e498fb5dc32 made the CPMD accounting
(slightly) less pessimistic (for single-task task sets). This changes
the expected outcome in the end-to-end test. This patch updates the
reference values accordingly.