Fix regression when a contextmanager yields a generic

[asottile]

Description

resolves #11852

I had to back out some of the tests added in the patch which regressed this behaviour -- once I had "fixed" them they were identical to existing tests

I added a new test to cover the behaviour that had regressed. I had to simplify it a bit from the one in the issue due to missing fixtures

Test Plan

automated tests in the patch itself -- the added test fails on master:

___________________ testContextManagerReturnsGenericFunction ___________________
[gw2] linux -- Python 3.7.12 /tmp/mypy/.tox/py37/bin/python
data: /tmp/mypy/test-data/unit/check-default-plugin.test:45:
../mypy/mypy/test/testcheck.py:140: in run_case
    self.run_case_once(testcase)
../mypy/mypy/test/testcheck.py:227: in run_case_once
    assert_string_arrays_equal(output, a, msg.format(testcase.file, testcase.line))
E   AssertionError: Unexpected type checker output (/tmp/mypy/test-data/unit/check-default-plugin.test, line 45)
----------------------------- Captured stderr call -----------------------------
Expected:
  main:26: note: Revealed type is "builtins.list[builtins.int*]" (diff)
Actual:
  main:24: error: Need type annotation for "m"  (diff)
  main:26: note: Revealed type is "builtins.list[Any]" (diff)

Alignment of first line difference:
  E: main:26: note: Revealed type is "builtins.list[builtins.int*]"
  A: main:24: error: Need type annotation for "m"
           ^

[JukkaL]

Thanks for the fix! Looks good.