kokoro: Increase xds-k8s timeout to 3 hours
kokoro: Increase xds-k8s timeout to 3 hours

The addition of the authz tests in 0d345721 is causing the tests to
exceed their timeout. By itself, the authz test takes about an hour in
this environment. Before the authz tests, xds-k8s was taking an hour
and a half.

grpc/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,grpc/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java