tests: move all tests to test/

tests: move all tests to test/

Instead of having some testing code interleaved with bup's
real code in lib/bup/test/ and lib/bup/t/ move everything
related to testing into test/, which is also renamed from
just t/.

This aligns with pytest for future changes, and, almost more
importantly, cleans up the lib/ directory to only have the
operational code.

The latter is necessary to properly 'libify' bup in order to
be able to use tox properly to test with different versions
of python in venv later.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
[rlb@defaultvalue.org: also move utilities like compare-trees to dev/;
 make test/lib a test-specific module tree and have wvtest.py include
 it; use test/ext/ and test/int/ (external and internal) as the test
 dir names.]

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>