prepare tests of Crispy.spy_into_instances
rename into Crispy.spy_into_instances
(WIP) trying to more programatic testing
shuffle method calls for test
Revert "shuffle method calls for test"

This reverts commit d71ea8941cee2f282abe1ea476603b9a2be03e1d.
simplify the first test again
sprout Spy::Target class to abstract codes to prepending spy features.
move code to prepend spy features, to spy_mixin
(WIP) add class spy
rename to execute tests without ArgumentError or NotImplementedError

but it seems to cause segfault
set class_spy into ClassSpy's class instance variable.

but I have segfault again
add debug codes before the segfault happenes
add more codes to reproduce segfault more.
Revert "add more codes to reproduce segfault more."

This reverts commit a56586e48bf3a88dd9d19068d53fdb67f3e2faac.
remove debug code
use instance method to retreive spy.

because BasicObject doesn't have `class` method, can't refer the class
when spying some message.
don't spy __CRISPY_SPY__ by mistake.

it happenes when doing spy_into and spy_into_instance against an object
of the same class.
add RecievedMessageWithReceiver
forgot to require dependent libraries of ClassSpy
test only received_messages (without receiver) so far.

tenporarily stop to test received_messages_with_receiver because it
contains the subject class's instances.
and the subject class inherits BasicObject,
which makes it difficult to compare by == method.