Add test-comparative-split-join to test against another bup

Add test-comparative-split-join to test against another bup

Add a test that will test the split/join behavior (including a
detailed repository/packfile comparison) with randomized data against
another version of bup, provided BUP_TEST_OTHER_BUP is set to the path
of a bup executable.

Among other things, this should provide a top-level backstop with
respect to changes in the hashsplitter.

Use this to test against the default debian and homebrew bup packages
in cirrus.

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