[testsuite] Added option to use different encodings for the output

tests/doc.md

@@ -274,6 +274,45 @@ This section describes more advanced features that are probably not necessary
 the "standard" usage of the test suite.
 
 
+### Using a different output encoding
+
+The test suite will try to interpret the program's output as utf-8 encoded
+strings and if that fails it will try the `iso-8859-1` encoding (also know as
+`latin-1`).
+
+If the tested program outputs characters in another encoding then it can be
+supplied as the `encodings` parameter in each test case:
+``` python
+# -*- coding: utf-8 -*-
+
+import system_tests
+
+
+class AnInformativeName(metaclass=system_tests.CaseMeta):
+
+    encodings = ['ascii']
+
+    filename = "invalid_input_file"
+    commands = [
+	    "$binary -c $import_file -i $filename"
+	]
+    retval = ["$abort_exit_value"]
+    stdout = ["Reading $filename"]
+    stderr = [
+        """$abort_error
+error in $filename
+"""
+    ]
+```
+
+The test suite will try to decode the program's output with the provided
+encodings in the order that they appear in the list. It will select the first
+encoding that can decode the output successfully. If no encoding is able to
+decode the program's output, then an error is raised. The list of all supported
+encodings can be found
+[here](https://docs.python.org/3/library/codecs.html#standard-encodings).
+
+
 ### Creating file copies
 
 For tests that modify their input file it is useful to run these with a

tests/system_tests.py

@@ -477,8 +477,26 @@ def test_run(self):
         got_stdout, got_stderr = proc.communicate()
         t.cancel()
 
-        processed_stdout = _process_output_post(got_stdout.decode('utf-8'))
-        processed_stderr = _process_output_post(got_stderr.decode('utf-8'))
+        processed_stdout = None
+        processed_stderr = None
+        for encoding in self.encodings:
+            try:
+                processed_stdout = _process_output_post(
+                    got_stdout.decode(encoding)
+                )
+                processed_stderr = _process_output_post(
+                    got_stderr.decode(encoding)
+                )
+            except UnicodeError:
+                pass
+            else:
+                break
+        if processed_stdout is None or processed_stderr is None:
+            raise ValueError(
+                "Could not decode the output of the command '{!s}' with the "
+                "following encodings: {!s}"
+                .format(command, ','.join(self.encodings))
+            )
 
         if _debug_mode:
             print(
@@ -506,6 +524,10 @@ class Case(unittest.TestCase):
     #: maxDiff set so that arbitrarily large diffs will be shown
     maxDiff = None
 
+    #: list of encodings that are used to decode the test program's output
+    #: the first encoding that does not raise a UnicodeError is used
+    encodings = ['utf-8', 'iso-8859-1']
+
     @classmethod
     def setUpClass(cls):
         """