"core/veyron/tools": Test must run under agent

This Part 2 of a series of changes to make go tests self-contained
and in particular run under an agent.
See: https://vanadium-review.googlesource.com/#/c/1931/

Change-Id: I956a15d7612e16d369c5b4882fa413c7c62aed04
diff --git a/tools/servicerunner/servicerunner_test.go b/tools/servicerunner/servicerunner_test.go
index 524f277..9ab0a1e 100644
--- a/tools/servicerunner/servicerunner_test.go
+++ b/tools/servicerunner/servicerunner_test.go
@@ -11,9 +11,12 @@
 	"os/exec"
 	"path"
 	"testing"
+
+	"v.io/core/veyron/lib/testutil"
 )
 
 func TestMain(t *testing.T) {
+	testutil.UnsetPrincipalEnvVars()
 	tmpdir, err := ioutil.TempDir("", "servicerunner_test")
 	if err != nil {
 		t.Fatal(err)