This change wires the `gotestsum` seam into the actual CI and ensures
that all tests _run_ and _pass_ using a new "gate" that verifies the
JUnit artifacts they produce. Specifically, the new gate works as
follows:
1. _registers_ that a test _should_ run and produce an artifact
2. _verifies_ that the artifact was produced and _all_ tests passed
(none were skipped).
The registration phase happens when a test is run in CI and requires
that the test be provided a `E2E_JUNIT_FILE` env variable.
Partial work for #1871
- [x] Tests pass
```
Run make build-junittool
make build-junittool
bin/junittool verify -manifest "${ARTIFACTS}/expected-junit.txt"
shell: /usr/bin/bash -e {0}
env:
E2E_ATENET_DATAPLANE: agentgateway
ARTIFACTS: /home/runner/work/substrate/substrate/_artifacts
go -C tools/junittool build -o
/home/runner/work/substrate/substrate/bin//junittool .
FILE TESTS FAILURES ERRORS SKIPPED
/home/runner/work/substrate/substrate/_artifacts/e2e-gvisor.xml 84 0 0 4
/home/runner/work/substrate/substrate/_artifacts/e2e-microvm.xml 84 0 0
5
/home/runner/work/substrate/substrate/_artifacts/e2e-mitm.xml 1 0 0 0
/home/runner/work/substrate/substrate/_artifacts/e2e-mitm-microvm.xml 1
0 0 0
/home/runner/work/substrate/substrate/_artifacts/e2e-networking-mitm.xml
11 0 0 1
/home/runner/work/substrate/substrate/_artifacts/e2e-networking-mitm-microvm.xml
11 0 0 1
TOTAL
```
- [x] Appropriate changes to documentation are included in the PR
