Make parallel_tests.sh build the kernel before running the tests.

This ensures that the kernel being tested exists and is is
up-to-date.

Test: ../net_test/parallel_tests.sh 20 2
Test: ../net_test/run_net_test.sh --builder all_tests.sh
Test: ../net_test/run_net_test.sh --norun
Test: ../net_test/run_net_test.sh multinetwork_test.py
Bug: 31119353
Change-Id: I40c298a089beddf9127a0e3f9da173082a68d65d