coalaTest: Add test preventing unknown warnings

Currently the test_coala_with_color and _without_color
tests fail with little indication of what is happening,
especially as the first warnings messages are related to
the .coafile 'Default' section.  The reported error is
trimmed, preventing the real cause of the error from
being seen in the logs.  It can be hiding a backtrace.

This introduces a new test which explicitly checks for
the two types of warnings which are currently happening,
being the 'Default' section and missing bears warnings.
These are happening because the repository .coafile is
being loaded instead of only using CLI arguments.

Then any additional warnings are considered to be an
error, and the entire output is show, so the reader
can see what is causing the problem.

Related to coala#5390

tests/coalaTest.py

@@ -1,4 +1,5 @@
 import os
+import re
 import sys
 import unittest
 import unittest.mock
@@ -344,6 +345,38 @@ def test_logged_error_causes_non_zero_exitcode(self):
 
             assert exitcode == 1
 
+    def test_coala_no_unexpected_warnings(self):
+        with bear_test_module(), \
+                prepare_file(['#fixme'], None) as (lines, filename):
+            retval, stdout, stderr = execute_coala(
+                coala.main, 'coala')
+            errors = filter(bool, stderr.split('\n'))
+            errors = list(errors)
+
+            unexpected = errors.copy()
+
+            expected = [
+                err for err in unexpected
+                if "Implicit 'Default' section inheritance" in err]
+            self.assertNotEqual([], expected)
+            # Filter them out
+            unexpected = [err for err in unexpected if err not in expected]
+
+            expected = [
+                err for err in errors
+                if re.search("No bears matching '.*' were installed", err)]
+            self.assertNotEqual([], expected)
+            # Filter them out
+            unexpected = [err for err in unexpected if err not in expected]
+
+            # Every error message must be about 'fixme'
+            unexpected = [err for err in errors if '#fixme' not in err]
+            self.assertEqual([], unexpected)
+            self.assertEqual(
+                retval, 0,
+                'coala must return zero when there are no errors;'
+                ' errors={errors}'.format(errors=list(errors)))
+
     def test_coala_with_color(self):
         with bear_test_module(), \
                 prepare_file(['#fixme'], None) as (lines, filename):