Some refactoring of mounts

- Separate FileMount into separate FileMount and WritableFileMount
   classes. This separates the (relatively simple) read-only code from
   the (soon to be even more complex) read/write code.

   It also allows you to create read-only mounts which don't bother with
   filesystem accounting, which is nice.

 - Make openForWrite/openForAppend always return a SeekableFileHandle.
   Appendable files still cannot be seeked within, but that check is now
   done on the FS side.

 - Refactor the various mount tests to live in test contract interfaces,
   allowing us to reuse them between mounts.

 - Clean up our error handling a little better. (Most) file-specific code
   has been moved to FileMount, and ArchiveMount-derived classes now
   throw correct path-localised exceptions.

tools/parse-reports.py

@@ -98,7 +98,7 @@ def _parse_junit_file(path: pathlib.Path):
 
             print("::group::Full error message")
             print(full_message)
-            print("::endgroup")
+            print("::endgroup::")
 
 
 def parse_junit() -> None: