Setup framework for running tests

[nginyc]

There are no tests for Rafiki, making it hard to determine whether there were any regressions.

We would want a single line test command and a framework to add/modify tests, which would reliably ensure that our code works as intended when changes are made.

[nudles]

I suggest to go top-down manner to save time of doing test for very small functions. e.g.,

def funcA():
    funcB()
    funcC()

def funcB():

def funcC():

We do test for funcA().

[nginyc]

ok understood. i will mainly add end-to-end tests e.g. full train-inference flow