2685_warn_undeclared: add test for warn.undeclared.

tests/errors/w_undeclared.pyx

+# cython: warn.undeclared=True
+# mode: error
+# tag: werror
+
+def foo():
+    a = 1
+    return a
+
+cdef class Bar:
+    cdef int baz(self, a):
+        res = 0
+        for i in range(3):
+            res += i
+        return res
+
+_ERRORS = """
+6:4: implicit declaration of 'a'
+11:8: implicit declaration of 'res'
+12:12: implicit declaration of 'i'
+"""