selftests: openvswitch: Bump timeout to 15 minutes.

We found that since some tests rely on the TCP SYN timeouts to cause flow
misses, the default test suite timeout of 45 seconds is quick to be
exceeded.  Bump the timeout to 15 minutes.
Signed-off-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240702132830.213384-2-aconole@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/openvswitch/settings

+timeout=900