pkgix: Allows relative prefixes which begin with -

Signed-off-by: Marco Elver <me@marcoelver.com>
tests: Cause error on log-mismatch, extend simple

Causes error on log-mismatch. Extend the simple test case with
avail, update and list commands.

Signed-off-by: Marco Elver <me@marcoelver.com>