Add CF testcases for generators and with statement

tests/errors/w_uninitialized_generators.pyx

+# cython: warn.maybe_uninitialized=True
+# mode: error
+# tag: werror
+
+def unbound_inside_generator(*args):
+    for i in args:
+        yield x
+        x = i + i
+
+_ERRORS = """
+7:15: local variable 'x' might be referenced before assignment
+"""

tests/errors/w_uninitialized_with.pyx

+# cython: warn.maybe_uninitialized=True
+# mode: error
+# tag: werror
+
+def with_no_target(m):
+    with m:
+        print a
+        a = 1
+
+def unbound_manager(m1):
+    with m2:
+        pass
+    m2 = m1
+
+def with_target(m):
+    with m as f:
+        print(f)
+
+_ERRORS = """
+7:15: local variable 'a' referenced before assignment
+11:11: local variable 'm2' referenced before assignment
+"""