upgrade_test: ignore backtrace of stall
upgrade_test: ignore backtrace of stall

The prompt of all backtrace followed by stall is `Backtrace:`, others are
`backtrace:`.

Some 3.1 upgrade_test failed for finding DB errors (backtrace) in log, but it's
just one backtrace from stall (500 ms). Let's ignore backtrace of stall in
final log checking, we have effect checking for stall log itself.

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests