To solve test cases that fail:

Even_loop() fixture properly closes the loop (but as explained below this is not enough to reset behaviour):
```
@pytest.fixture
def event_loop(request):
    """Create an instance of the default event loop for each test case."""
    loop = asyncio.get_event_loop_policy().new_event_loop()
    yield loop
    loop.close()
```
Subsequent calls to asyncio.get_event_loop() returns the previous closed loop, instead of providing a new loop.
This is due to (https://github.com/python/cpython/blob/3.8/Lib/asyncio/events.py#L634) variable _set_called is True, and so even when loop is None, then no new_event_loop is created [I really don´t know the reason of this _set_called behaviour].

The best solution I have found is to set_event_loop_policy(None). This completely resets global variable and so any subsequent call to asyncio.get_event_loop() provides a new event_loop.
I have included this in the pytest hook (pytest_fixture_post_finalizer), that is called after fixture teardowns.

Note: Same behaviour can be done modifying event_loop() fixture, but I have discarded it as this would force users that have overwritten even_loop fixture to modify its implementation:
```
@pytest.fixture
def event_loop(request):
    """Create an instance of the default event loop for each test case."""
    loop = asyncio.get_event_loop_policy().new_event_loop()
    yield loop
    loop.close()
	asyncio.set_event_loop_policy(None)
```

Author: alblasco

pytest_asyncio/plugin.py

@@ -72,6 +72,14 @@ def get_and_strip_from(self, name, data_dict):
             del data_dict[name]
         return result
 
+@pytest.hookimpl(trylast=True)
+def pytest_fixture_post_finalizer(fixturedef, request):
+    """Called after fixture teardown"""
+    if fixturedef.argname == "event_loop":
+        # Set empty loop policy, so that subsequent get_event_loop() provides a new loop
+        asyncio.set_event_loop_policy(None)
+
+
 
 @pytest.hookimpl(hookwrapper=True)
 def pytest_fixture_setup(fixturedef, request):