Test that tor correctly handles zero-length keys

Check that tor generates new keys, and overwrites the empty key files.
Test that tor generates new keys when keys are missing (existing
behaviour).
Test that tor does not overwrite key files that already contain data
(existing behaviour).

Tests fixes to bug 13111.

changes/bug13111-generate-keys-on-empty-file

@@ -1,17 +1,23 @@
-  o Minor bugfixes
+  o Minor bugfixes (file handling):
     - Stop failing when key files are zero-length. Instead, generate new
       keys, and overwrite the empty key files.
-      Fixes bug 13111.
+      Fixes bug 13111. Patch by "teor".
     - Stop generating a fresh .old RSA key file when the .old file is missing.
-      Fixed as part of bug 13111.
     - Avoid overwriting .old key files with empty key files.
-      Fixed as part of bug 13111.
     - Stop crashing when a NULL filename is passed to file_status().
-      Fixed as part of bug 13111.
+      Fixed as part of bug 13111. Patches by "teor".
 
-  o Minor enhancements:
+  o Minor enhancements (file handling):
     - Skip loading zero-length extra info store, router store, stats, state,
       and key files.
-      Implemented with bug 13111.
     - Return FN_ERROR when a zero-length filename is passed to file_status().
-      Fixed as part of bug 13111.
+      Fixed as part of bug 13111. Patches by "teor".
+
+  o Minor enhancements (testing):
+    - Test that tor does not fail when key files are zero-length.
+      Check that tor generates new keys, and overwrites the empty key files.
+    - Test that tor generates new keys when keys are missing (existing
+      behaviour).
+    - Test that tor does not overwrite key files that already contain data
+      (existing behaviour).
+      Tests bug 13111. Patch by "teor".