Check for warnings in some C++ tests.

tests/compile/cpp_class_redefinition.pyx

-# tag: cpp
 # mode: compile
+# tag: cpp, warnings
 
 cdef extern from "templates.h":
     cdef cppclass TemplateTest1[T]:

tests/run/cpp_class_redef.pyx

-# tag: cpp
+# mode: run
+# tag: cpp, warnings
 
-# This gives a warning, but should not give an error.
+# This gives a warning about the previous .pxd definition, but should not give an error.
 cdef cppclass Foo:
     int _foo
     int get_foo():
@@ -20,3 +21,8 @@ def test_Foo(n):
         return foo.get_foo()
     finally:
         del foo
+
+
+_WARNINGS = """
+5:5: 'Foo' already defined  (ignoring second definition)
+"""

tests/run/cpp_template_functions.pyx

-# tag: cpp
+# mode: run
+# tag: cpp, warnings
 
 cimport cython
 from libcpp.pair cimport pair