dpkg: add ptest support

dpkg: add ptest support

Added it to slow tests, as it takes betwen 165 and 190 seconds on my
machine to execute (qemux86-64 + kvm).

The test folder's Makefile contains a list of passing, failing and manual
tests. By default, only the expected-to-pass tests are executed by the
Makefile (unless magic environment variable is set).

The run-ptest script mimics the default behavior of executing the
expected-to-pass tests, however they are executed one by one, instead of
running them as one batch - that way it is easier to determine exactly
which tests pass and which fail.

One other thing that might worth a note, is that the tests folder that needs to be
installed contains a number of subfolders called "DEBIAN". When packaging them
at least with rpm, these folders are omitted from the package.
However these are essential for the tests, as they contain test data. As a
workaround, these folders are renamed during installation to DEBIAN-ptest,
and before execution the run-ptest script restores their names.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>