e2e: Cover RunnerDeployment (#668)

Previously the E2E test suite covered only RunnerSet. This refactors the existing E2E test code to extract the common test structure into a `env` struct and its methods, and use it to write two very similar tests, one for RunnerSet and another for RunnerDeployment.

testing/getenv.go

@@ -0,0 +1,16 @@
+package testing
+
+import (
+	"os"
+	"testing"
+)
+
+func Getenv(t *testing.T, name string) string {
+	t.Helper()
+
+	v := os.Getenv(name)
+	if v == "" {
+		t.Fatal(name + " must be set")
+	}
+	return v
+}