Add tests for passing a non const variable as a const qualified

function parameter.

tests/run/non_const_as_const_arg.pyx

+cdef double f(const double a, const double b, const double c):
+    return a + b - c
+
+def test_non_const_as_const_arg():
+    """
+    >>> test_non_const_as_const_arg()
+    1.0
+    """
+    cdef double a = 1., b = 1., c = 1.
+    return f(a, b, c)