[mypyc] Display IR on annotate test failure

[JukkaL]

This makes it easier to figure out why a test is failing.

Example output on failure:

=================================== FAILURES ====================================
________________________ testAnnotateTwoOperationsOnLine ________________________
data: /Users/jukka/src/mypy/mypyc/test-data/annotate-basic.test:18:
Failed: Invalid source code output (/Users/jukka/src/mypy/mypyc/test-data/annotate-basic.test, line 18)
----------------------------- Captured stdout call ------------------------------
Generated IR:

def f(x):
    x :: object
    r0 :: str
    r1, r2, r3 :: object
L0:
    r0 = 'foo'
    r1 = CPyObject_GetAttr(x, r0)
    r2 = object 1
    r3 = PyNumber_Add(r1, r2)
    return r3
----------------------------- Captured stderr call ------------------------------
Expected:
  main:2: Get non-native attribute "foo". Generic "+" operation.x (diff)
Actual:
  main:2: Get non-native attribute "foo". Generic "+" operation. (diff)

Alignment of first line difference:
  E: ...Generic "+" operation.x
  A: ...Generic "+" operation.
                              ^
Update the test output using --update-data (implies -n0; you can additionally use the -k selector to update only specific tests)