add test for utility code inclusion in conversion of external structs

tests/run/struct_conversion_extern.pyx

+"""
+Note: this tests if the necessary utility code is included in the module env,
+despite potentially being already created before.
+"""
+
+cdef extern from "struct_conversion_extern_header.h":
+    cdef struct my_date_t:
+        int year
+        int month
+        int day
+
+
+def test_extern_struct():
+    """
+    >>> test_extern_struct()
+    [('day', 24), ('month', 6), ('year', 2000)]
+    """
+    cdef my_date_t day = my_date_t(year=2000, month=6, day=24)
+    cdef object d = day
+    assert type(d) is dict
+    assert d == day
+    return sorted(day.items())

tests/run/struct_conversion_extern_header.h

+struct my_date_t {
+    int year;
+    int month;
+    int day;
+};