Fix trace in DataCollectionRequestSender.cs (#15025)

String join:

```
..., vstest.console.exe, DataCollectionRequestSender.SendBeforeTestRunStartAndGetResult: Send BeforeTestRunStart message with settingsXml <RunSettings>...</RunSettings> and sources System.Collections.Generic.List`1[System.String]: 
```

Author: stan-sz

src/Microsoft.TestPlatform.CommunicationUtilities/DataCollectionRequestSender.cs

@@ -103,7 +103,7 @@ public void SendTestHostLaunched(TestHostLaunchedPayload testHostLaunchedPayload
         var isDataCollectionStarted = false;
         BeforeTestRunStartResult? result = null;
 
-        EqtTrace.Verbose("DataCollectionRequestSender.SendBeforeTestRunStartAndGetResult: Send BeforeTestRunStart message with settingsXml {0} and sources {1}: ", settingsXml, sources.ToString());
+        EqtTrace.Verbose("DataCollectionRequestSender.SendBeforeTestRunStartAndGetResult: Send BeforeTestRunStart message with settingsXml {0} and sources {1}: ", settingsXml, string.Join(" ", sources));
 
         var payload = new BeforeTestRunStartPayload
         {