Add test for ticket #1888: mismatching argument names between .pxd and .pyx...

Add test for ticket #1888: mismatching argument names between .pxd and .pyx files are (not great but) actually ok, but the callers must always see the "public" pxd names, and the implementation must see and use the names given by the implementation.

tests/run/pxd_argument_names.srctree

+# node: run
+# ticket: gh1888
+
+PYTHON setup.py build_ext --inplace
+PYTHON -c "import a; a.test()"
+
+######## setup.py ########
+
+from Cython.Build.Dependencies import cythonize
+
+from distutils.core import setup
+
+setup(
+    ext_modules=cythonize("a.pyx"),
+)
+
+######## a.pxd ########
+
+cdef int do_stuff(int foo) except -1
+
+######## a.pyx ########
+
+cdef int do_stuff(int bar) except -1:
+    if bar == 0:
+        raise ValueError()
+    return bar
+
+
+cdef call_do_stuff(int x):
+    # The keyword argument name is surprising, but actually correct.
+    # The caller signature is defined by the .pxd file, not the implementation.
+    return do_stuff(foo=x)
+
+
+def test():
+    assert do_stuff(1) == 1
+    assert do_stuff(foo=1) == 1
+    assert call_do_stuff(1) == 1
+
+    try:
+        do_stuff(0)
+    except ValueError:
+        pass
+    else:
+        assert False, "exception not raised"
+
+    try:
+        call_do_stuff(0)
+    except ValueError:
+        pass
+    else:
+        assert False, "exception not raised"