[link-quality] have LinkQualityInfo inherit from IntsanceLocatorInit (#4569)

tests/unit/test_link_quality.cpp

@@ -34,6 +34,8 @@
 
 namespace ot {
 
+static ot::Instance *sInstance;
+
 enum
 {
     kMaxRssValue = 0,
@@ -89,6 +91,10 @@ void TestLinkQualityData(RssTestData aRssData)
     int8_t          rss, ave, min, max;
     size_t          i;
 
+    sInstance = testInitInstance();
+    VerifyOrQuit(sInstance != NULL, "Null instance");
+    linkInfo.Init(*sInstance);
+
     printf("- - - - - - - - - - - - - - - - - -\n");
     linkInfo.Clear();
     min = kMinRssValue;