test: Add feature for .rels file count

Make sure that a .rels parts is not written for any part that has no
outgoing relationships.

features/prs-open-save.feature

@@ -26,3 +26,4 @@ Feature: Round-trip a presentation
      Given a presentation with external relationships
       When I save and reload the presentation
       Then the external relationships are still there
+       And the package has the expected number of .rels parts

features/steps/presentation.py

@@ -1,12 +1,9 @@
 # encoding: utf-8
 
-"""
-Gherkin step implementations for presentation-level features.
-"""
-
-from __future__ import absolute_import
+"""Gherkin step implementations for presentation-level features."""
 
 import os
+import zipfile
 
 from behave import given, when, then
 
@@ -181,6 +178,13 @@ def then_ext_rels_are_preserved(context):
     assert rel.target_ref == "https://github.com/scanny/python-pptx"
 
 
+@then("the package has the expected number of .rels parts")
+def then_the_package_has_the_expected_number_of_rels_parts(context):
+    with zipfile.ZipFile(saved_pptx_path, "r") as z:
+        member_count = len(z.namelist())
+    assert member_count == 18, "expected 18, got %d" % member_count
+
+
 @then("the slide height matches the new value")
 def then_slide_height_matches_new_value(context):
     presentation = context.presentation