t/t9700/test.pl: don't access private object members, use public access methods

t/t9700/test.pl: don't access private object members, use public access methods

This test is accessing private object members of the Test::More and
Test::Builder objects.  Older versions of Test::More did not implement
these variables using a hash.

My system complains as follows:

   Can't coerce array into hash at <snip>/t/t9700/test.pl line 13.
   BEGIN failed--compilation aborted at <snip>/t/t9700/test.pl line 15.

There are public access methods available for retrieving and setting these
variables, so let's use them instead.

Signed-off-by: Brandon Casey <redacted>
Acked-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>