Introspect logging testing more

This adds some fixtures to ensure the log output is what we expect,
along with ensuring we test the operation of the debug environment
variables.

Change-Id: I01c36299539b15a633b9307da5a348a5ae4e2563

diskimage_builder/tests/test_loggingconfig.py

@@ -12,6 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import fixtures
 import logging
 import testtools
 
@@ -21,9 +22,28 @@ from diskimage_builder import logging_config
 class TestLoggingConfig(testtools.TestCase):
 
     def test_defaults(self):
+        self.out = self.useFixture(fixtures.StringStream('stdout'))
+        self.useFixture(
+            fixtures.MonkeyPatch('sys.stdout', self.out.stream))
+
+        self.err = self.useFixture(fixtures.StringStream('stderr'))
+        self.useFixture(
+            fixtures.MonkeyPatch('sys.stderr', self.err.stream))
+
+        self.useFixture(fixtures.EnvironmentVariable('DIB_DEBUG_TRACE', '1'))
+
         logging_config.setup()
         log = logging.getLogger(__name__)
         log.debug("Debug Message")
+        self.assertIn("Debug Message", self.err._details["stderr"].as_text())
+        # The follow two are looking for the function name / file name
+        # suffix we log only for debug messages
+        self.assertIn("test_defaults", self.err._details["stderr"].as_text())
+        self.assertIn("test_loggingconfig.py",
+                      self.err._details["stderr"].as_text())
         log.info("Info Message")
+        self.assertIn("Info Message", self.err._details["stderr"].as_text())
         log.warning("Warning Message")
+        self.assertIn("Warning Message", self.err._details["stderr"].as_text())
         log.error("Error Message")
+        self.assertIn("Error Message", self.err._details["stderr"].as_text())