I already reported the issue in the CL where it was introduced. /cc @stevenh @bradfitz Creating this issue for posterity, so I can reference it in the CL I'm about to send.

TestEnvVars contains this code:

for i, cn := range tc.cns {
	if i > len(r.certs) {
		t.Errorf("missing cert %v @ %v", cn, i)
	} else if r.certs[i].Subject.CommonName != cn {
		// ...
	}
}

It should have i >= len(r.certs) instead, otherwise r.certs[i] in the else if will panic when i == len(r.certs).

This panic happens when the test fails due to the returned number of certificates (r.certs) being less than expected by test case (tc.cns).

CL upcoming. Edit: Sent CL 46715.