cmd: Making test watch-mode tests more robust (#6019)

* Waiting for output buffer to contain expected data rather than making exact matches on the entire content.
* Not aborting watcher on encountered errors

Signed-off-by: Johan Fylling <johan.dev@fylling.se>

util/test/tempus.go

@@ -0,0 +1,22 @@
+// Copyright 2023 The OPA Authors.  All rights reserved.
+// Use of this source code is governed by an Apache2
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"testing"
+	"time"
+)
+
+func Eventually(t *testing.T, timeout time.Duration, f func() bool) bool {
+	t.Helper()
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		if f() {
+			return true
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	return false
+}