selftest: add an option to specify the test list

selftest: add an option to specify the test list

This can be used to override the default test lists used by
`make test` and `make perftest`. This tests can either be
programmatically generated (as is done for the defaults -- see
selftest/tests.py for an example), or from a static list. For the
generated lists, append a pipe symbol:

    make test TEST_LIST='/bin/sh /tmp/tests.sh|'

and omit the pipe for a static list:

    make test TEST_LIST='/tmp/tests.txt'

There are likely other useful modes of operation -- see `perldoc open`
for the wondrous details.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>