Update name of test/htest in doc/devnotes.rst

Update the docs after the rename of 'test/hs/test' to
'test/hs/htest'. This must have been forgotten in 3e16567e.

Signed-off-by: Yiannis Tsiouris <gtsiour@softlab.ntua.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>