Move htest/ files under the test/ tree

htest/data becomes test/data/htools (basically reverting commit
8feabc89), and htest/* becomes test/hs/*.

Most changes beside the rename are trivial s/…/…, with the exception
of autotools/run-in-tempdir, which needed some more changes now that
test/ is not just Python files:

- test/py is still being copied
- test/hs moves from individual symlinks to entire dir symlink
- test/data is symlinked in its entirety

Checked with make distcheck, pep8 and pylint, so at least VPATH builds
are OK.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>