Isolate the code that resolves the fixturefunc to a separate function

pytest_fixture_setup was somewhat convoluted because it was trying
to do too many things.

src/_pytest/fixtures.py

@@ -899,15 +899,10 @@ class FixtureDef(object):
         )
 
 
-def pytest_fixture_setup(fixturedef, request):
-    """ Execution of fixture setup. """
-    kwargs = {}
-    for argname in fixturedef.argnames:
-        fixdef = request._get_active_fixturedef(argname)
-        result, arg_cache_key, exc = fixdef.cached_result
-        request._check_scope(argname, request.scope, fixdef.scope)
-        kwargs[argname] = result
-
+def resolve_fixture_function(fixturedef, request):
+    """Gets the actual callable that can be called to obtain the fixture value, dealing with unittest-specific
+    instances and bound methods.
+    """
     fixturefunc = fixturedef.func
     if fixturedef.unittest:
         if request.instance is not None:
@@ -921,6 +916,19 @@ def pytest_fixture_setup(fixturedef, request):
             fixturefunc = getimfunc(fixturedef.func)
             if fixturefunc != fixturedef.func:
                 fixturefunc = fixturefunc.__get__(request.instance)
+    return fixturefunc
+
+
+def pytest_fixture_setup(fixturedef, request):
+    """ Execution of fixture setup. """
+    kwargs = {}
+    for argname in fixturedef.argnames:
+        fixdef = request._get_active_fixturedef(argname)
+        result, arg_cache_key, exc = fixdef.cached_result
+        request._check_scope(argname, request.scope, fixdef.scope)
+        kwargs[argname] = result
+
+    fixturefunc = resolve_fixture_function(fixturedef, request)
     my_cache_key = request.param_index
     try:
         result = call_fixture_func(fixturefunc, request, kwargs)