standalone_test: Be consistent among logs and --list output

Create a method that inspects the cartesian file for all
extra filters that the test runner add, and display the
same information for both the output of --list-tests and
the debug log. For example:

"""
Tests produced by config file /path/to/tests.cfg

The full test list was modified by the following:

Filters applied:
    only JeOS.17.64
    only smp2
    only qcow2
    only virtio_net
    only virtio_blk

Different guest OS have different test lists

Assignments applied:
    mem = 512
    qemu_binary = /bin/qemu-kvm
    qemu_img_binary = /bin/qemu-img
    qemu_io_binary = /bin/qemu-io
    nettype = user

Assignments override values previously set in the config file

1 balloon_check.base
...
"""

Hopefully that provides enough context information so
people get less confused on what the test runner is doing.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>