Add and modify tests to match readme for part 4

Add a test for calling foo_history reader after calling foo setter
exactly once. Rename existing test for calling foo_history reader after
calling foo setter twice to indicate specific to calling setter twice.
Modify test for remembering values to remember several values instead
of just one non-nil value.

These modifications make the tests match the examples given in the
readme, specifically that a history longer than 2 should work and that
the history should be [nil] after calling setter once.

Signed-off-by: Jonathan Mendez <37cn3t39yh@snkmail.com>