Add test for consistent random ordering

-- Fixed up during cherrypicking

tests/CMakeLists.txt

@@ -198,6 +198,9 @@ set_tests_properties(TagAlias PROPERTIES
   FAIL_REGULAR_EXPRESSION "0 matching test cases"
 )
 
+add_test(NAME RandomTestOrdering COMMAND ${PYTHON_EXECUTABLE}
+  ${CATCH_DIR}/tests/TestScripts/testRandomOrder.py $<TARGET_FILE:SelfTest>)
+
 if (CATCH_USE_VALGRIND)
     add_test(NAME ValgrindRunTests COMMAND valgrind --leak-check=full --error-exitcode=1 $<TARGET_FILE:SelfTest>)
     add_test(NAME ValgrindListTests COMMAND valgrind --leak-check=full --error-exitcode=1 $<TARGET_FILE:SelfTest> --list-tests --verbosity high)

tests/TestScripts/testRandomOrder.py

@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+
+import subprocess
+import sys
+import xml.etree.ElementTree as ET
+
+def list_tests(self_test_exe, tags, rng_seed):
+    cmd = [self_test_exe, '--reporter', 'xml', '--list-tests', '--order', 'rand',
+            '--rng-seed', str(rng_seed)]
+    tags_arg = ','.join('[{}]'.format(t) for t in tags)
+    if tags_arg:
+        cmd.append(tags_arg + '~[.]')
+    process = subprocess.Popen(
+            cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    stdout, stderr = process.communicate()
+    if stderr:
+        raise RuntimeError("Unexpected error output:\n" + process.stderr)
+
+    root = ET.fromstring(stdout)
+    result = [elem.text for elem in root.findall('./TestCase/Name')]
+
+    if len(result) < 2:
+        raise RuntimeError("Unexpectedly few tests listed (got {})".format(
+            len(result)))
+    return result
+
+def check_is_sublist_of(shorter, longer):
+    assert len(shorter) < len(longer)
+    assert len(set(longer)) == len(longer)
+
+    indexes_in_longer = {s: i for i, s in enumerate(longer)}
+    for s1, s2 in zip(shorter, shorter[1:]):
+        assert indexes_in_longer[s1] < indexes_in_longer[s2], (
+                '{} comes before {} in longer list.\n'
+                'Longer: {}\nShorter: {}'.format(s2, s1, longer, shorter))
+
+def main():
+    self_test_exe, = sys.argv[1:]
+
+    list_one_tag = list_tests(self_test_exe, ['generators'], 1)
+    list_two_tags = list_tests(self_test_exe, ['generators', 'matchers'], 1)
+    list_all = list_tests(self_test_exe, [], 1)
+
+    # First, verify that restricting to a subset yields the same order
+    check_is_sublist_of(list_two_tags, list_all)
+    check_is_sublist_of(list_one_tag, list_two_tags)
+
+    # Second, verify that different seeds yield different orders
+    num_seeds = 100
+    seeds = range(1, num_seeds + 1) # Avoid zero since that's special
+    lists = {tuple(list_tests(self_test_exe, [], seed)) for seed in seeds}
+    assert len(lists) == num_seeds, (
+            'Got {} distict lists from {} seeds'.format(len(lists), num_seeds))
+
+if __name__ == '__main__':
+    sys.exit(main())