Changes file for log test improvements

changes/bug19999_prep

+  o Minor features (unit tests):
+    - The unit tests now log all warning messages with the "BUG" flag.
+      Previously, they only logged errors by default. This change will
+      help us make our testing code more correct, and make sure that
+      we only hit this code when we mean to. This is preparatory work
+      for ticket 19999.
+    - Our unit testing code that captures log messages no longer prevents
+      them from being written out if the user asked for them (by passing
+      --debug or --info or or --notice --warn to the "test" binary). This
+      change will prevent us from missing unexpected log messages simply
+      because we were looking for others. Related to ticket 19999.
+    - Our link-handshake unit tests now check, that when invalid
+      handshakes fail, they fail with the error messages we
+      expected.
+
+  o Minor bugfixes (unit tests):
+    - The tor_tls_server_info_callback unit test no longer crashes when
+      debug-level logging is turned on. Fixes bug 20041; bugfix on
+      0.2.8.1-alpha.
+