fix: keep the log handler set by the brokerage test project

- only swap Lean's default ConsoleLogHandler for NUnitLogHandler
- brokerage test projects keep their file log for the whole run

Author: Romazes

Tests/Brokerages/BrokerageTests.cs

@@ -50,7 +50,11 @@ public abstract class BrokerageTests
         [SetUp]
         public void Setup()
         {
-            Log.LogHandler = new NUnitLogHandler();
+            // keep the log handler set by the test project, only replace Lean's default
+            if (Log.LogHandler is ConsoleLogHandler)
+            {
+                Log.LogHandler = new NUnitLogHandler();
+            }
 
             Log.Trace("");
             Log.Trace("");