Mock objects' checkpoint methods no longer check static expectations.

[asomers]

This behavior is more useful where static and regular methods are both
used in the same test. The old behavior was a relic from release 0.3.0
and before, when static methods did not yet have Context objects of
their own.

This commit also fixes intermittent test failures in
mock_generic_struct_with_generic_static_method and
mock_struct_with_static_method. Those tests were failing because the
mock objects' checkpoints from one test were unexpectedly interfering
with static methods expectations' from other tests.