t1308-config-set: fix a test that has a typo

t1308-config-set: fix a test that has a typo

Change test 'find value_list for a key from a configset' to redirect the
result to 'expect' instead of 'except' which was a typo.

With this change, the test case actually fails because it uses
`configset_get_value`. Clearly, this was intended to be
`configset_get_value_multi` since the test expects a list of values
instead of a single value, so let's fix that, too.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>