[CTS] Clarify CTS assert message for missing RTT AP support

Add a much more verbose message for the common failure case in which
the CTS is run without an AP which supports IEEE 802.11mc / RTT. That
may help speed up issue resolution.

Bug: 116849381
Test: atest WifiRttTest
Change-Id: I4ea3fbe17563f0087a013cbf48f7f601fec7e363

tests/cts/net/src/android/net/wifi/rtt/cts/WifiRttTest.java

@@ -16,11 +16,9 @@
 
 package android.net.wifi.rtt.cts;
 
-import android.content.IntentFilter;
 import android.net.wifi.ScanResult;
 import android.net.wifi.rtt.RangingRequest;
 import android.net.wifi.rtt.RangingResult;
-import android.net.wifi.rtt.WifiRttManager;
 
 import com.android.compatibility.common.util.DeviceReportLog;
 import com.android.compatibility.common.util.ResultType;
@@ -64,7 +62,10 @@ public class WifiRttTest extends TestBase {
 
         // Scan for IEEE 802.11mc supporting APs
         ScanResult testAp = scanForTestAp(NUM_SCANS_SEARCHING_FOR_IEEE80211MC_AP);
-        assertTrue("Cannot find test AP", testAp != null);
+        assertTrue(
+                "Cannot find any test APs which support RTT / IEEE 802.11mc - please verify that "
+                        + "your test setup includes them!",
+                testAp != null);
 
         // Perform RTT operations
         RangingRequest request = new RangingRequest.Builder().addAccessPoint(testAp).build();