kunit: increase KUNIT_LOG_SIZE to 2048 bytes

The s390 specific test_unwind kunit test has 39 parameterized tests. The
results in debugfs are truncated since the full log doesn't fit into 1500
bytes.
Therefore increase KUNIT_LOG_SIZE to 2048 bytes in a similar way like it
was done recently with commit "kunit: fix bug in debugfs logs of
parameterized tests". With that the whole test result is present.

Reported-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>