Fix unstable ItemChannelLinkResourceOSGiTest (openhab#2536)

New ItemChannelLinkProviders are added asynchronously to the ItemChannelLinkRegistry, which can cause this test to fail:

TEST org.openhab.core.io.rest.core.internal.link.ItemChannelLinkResourceOSGiTest#shouldReturnLinks() <<< ERROR:
Expected: a collection with size <3>
     but: collection size was <0>
java.lang.AssertionError:
Expected: a collection with size <3>
     but: collection size was <0>
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:6)
	at org.openhab.core.io.rest.core.internal.link.ItemChannelLinkResourceOSGiTest.shouldReturnLinks(ItemChannelLinkResourceOSGiTest.java:109)

Signed-off-by: Wouter Born <github@maindrain.net>
GitOrigin-RevId: 39dad81

itests/org.opensmarthouse.core.io.rest.core.tests/src/main/java/org/openhab/core/io/rest/core/internal/link/ItemChannelLinkResourceOSGiTest.java

@@ -42,6 +42,7 @@
 import org.openhab.core.thing.ChannelUID;
 import org.openhab.core.thing.link.ItemChannelLink;
 import org.openhab.core.thing.link.ItemChannelLinkProvider;
+import org.openhab.core.thing.link.ItemChannelLinkRegistry;
 import org.openhab.core.thing.link.ManagedItemChannelLinkProvider;
 
 import com.google.gson.JsonElement;
@@ -73,6 +74,7 @@ public class ItemChannelLinkResourceOSGiTest extends JavaOSGiTest {
     private UriInfo uriInfo;
     private HttpHeaders httpHeaders;
 
+    private ItemChannelLinkRegistry itemChannelLinkRegistry;
     private ItemChannelLinkResource itemChannelLinkResource;
     private ManagedItemChannelLinkProvider managedItemChannelLinkProvider;
 
@@ -83,6 +85,9 @@ public void beforeEach() {
         itemChannelLinkResource = getService(RESTResource.class, ItemChannelLinkResource.class);
         assertNotNull(itemChannelLinkResource);
 
+        itemChannelLinkRegistry = getService(ItemChannelLinkRegistry.class);
+        assertNotNull(itemChannelLinkRegistry);
+
         managedItemChannelLinkProvider = getService(ManagedItemChannelLinkProvider.class);
         assertNotNull(managedItemChannelLinkProvider);
 
@@ -93,6 +98,10 @@ public void beforeEach() {
         when(itemChannelLinkProvider.getAll()).thenReturn(List.of(link1, link2, link3));
         registerService(itemChannelLinkProvider);
 
+        waitForAssert(() -> {
+            assertThat(itemChannelLinkRegistry.getAll(), hasSize(3));
+        });
+
         UriBuilder uriBuilder = mock(UriBuilder.class);
         when(uriBuilder.build(any())).thenReturn(URI.create(""));
         uriInfo = mock(UriInfo.class);