app/test: filter out unavailable tests

Some tests can fail to run because they are not compiled.
It has been more visible recently when the PCI test became disabled
in the default configuration because of dependency on libarchive:
    PCI autotest:    Fail [Not found]

The autotest script catch them and do not count them as an error anymore:
    PCI autotest:    Skipped [Not Available]

The commands dump_ring and dump_mempool are removed from the autotest list
because they are some internal commands but not some registered tests.
Thus they cannot be parsed as available test commands.

Suggested-by: David Marchand <david.marchand@6wind.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>