umt autopkgtest should support testing on packages from a ppa upload

[Steve Beattie]

Currently, umt autopkgtest runs its tests on the pacakges in the ../binaries/ directory which it the directory that contains the locally built binary pacakges. That's useful, but it would also be useful if it run the test on the binary pacakges built in a ppa a la how umt compare-bin supports the --ppa option to run the binary package comparison. This ensures that any locally run autopkgtests are performed on the binaries that actually get installed on users' systems, not just the ones locally built.