Fix ClusterHealthResponsesTests condition (#49360)

Currently the condtion that is supposed to test creation of test instances with
multiple indices is never true because it compares Strings with an enum. This
changes it so the condition uses the enum constants instead.

Author: Christoph Büscher

server/src/test/java/org/elasticsearch/action/admin/cluster/health/ClusterHealthResponsesTests.java

@@ -113,7 +113,7 @@ protected ClusterHealthResponse doParseInstance(XContentParser parser) {
     protected ClusterHealthResponse createTestInstance() {
         int indicesSize = randomInt(20);
         Map<String, ClusterIndexHealth> indices = new HashMap<>(indicesSize);
-        if ("indices".equals(level) || "shards".equals(level)) {
+        if (ClusterHealthRequest.Level.INDICES.equals(level) || ClusterHealthRequest.Level.SHARDS.equals(level)) {
             for (int i = 0; i < indicesSize; i++) {
                 String indexName = randomAlphaOfLengthBetween(1, 5) + i;
                 indices.put(indexName, ClusterIndexHealthTests.randomIndexHealth(indexName, level));