Loki: Add backend healthcheck (#74330)

* add loki healthcheck

* remove `testDatasource` call

* remove unused error check

* change success message

* improve error messages

e2e/various-suite/loki-editor.spec.ts

@@ -4,8 +4,7 @@ const dataSourceName = 'LokiEditor';
 const addDataSource = () => {
   e2e.flows.addDataSource({
     type: 'Loki',
-    expectedAlertMessage:
-      'Unable to connect with Loki (Failed to call resource). Please check the server logs for more details.',
+    expectedAlertMessage: 'Unable to connect with Loki. Please check the server logs for more details.',
     name: dataSourceName,
     form: () => {
       e2e().get('#connection-url').type('http://loki-url:3100');

e2e/various-suite/loki-query-builder.spec.ts

@@ -5,8 +5,7 @@ const dataSourceName = 'LokiBuilder';
 const addDataSource = () => {
   e2e.flows.addDataSource({
     type: 'Loki',
-    expectedAlertMessage:
-      'Unable to connect with Loki (Failed to call resource). Please check the server logs for more details.',
+    expectedAlertMessage: 'Unable to connect with Loki. Please check the server logs for more details.',
     name: dataSourceName,
     form: () => {
       e2e().get('#connection-url').type('http://loki-url:3100');