Shorten cube iterator tests (SciTools#6041)

* Shorten cube iterator tests

* Small improvements and whatsnew

docs/src/whatsnew/latest.rst

@@ -155,6 +155,7 @@ This document explains the changes made to Iris for this release
    benchmark historic commits (especially older Python versions).
    (:pull:`5963`)
 
+#. `@bouweandela`_ made some tests for :func:`~iris.iterate.izip` faster. (:pull:`6041`)
 
 .. comment
     Whatsnew author names (@github name) in alphabetical order. Note that,

lib/iris/tests/__init__.py

@@ -192,6 +192,31 @@ def assert_masked_array_almost_equal(a, b, decimal=6, strict=False):
     )
 
 
+def assert_cml(cubes, reference_filename=None, checksum=True):
+    """Test that the CML for the given cubes matches the contents of
+    the reference file.
+
+    If the environment variable IRIS_TEST_CREATE_MISSING is
+    non-empty, the reference file is created if it doesn't exist.
+
+    Parameters
+    ----------
+    cubes :
+        Either a Cube or a sequence of Cubes.
+    reference_filename : optional, default=None
+        The relative path (relative to the test results directory).
+        If omitted, the result is generated from the calling
+        method's name, class, and module using
+        :meth:`iris.tests.IrisTest.result_path`.
+    checksum : bool, optional
+        When True, causes the CML to include a checksum for each
+        Cube's data. Defaults to True.
+
+    """
+    test = IrisTest()
+    test.assertCML(cubes, reference_filename, checksum)
+
+
 class IrisTest(unittest.TestCase):
     """A subclass of unittest.TestCase which provides Iris specific testing functionality."""