fakebts: Add the missing method needed for the LUTest

The LUTest created a BTS with a specific UnitId but the code was
not yet inside the OpenBSCTest class. Fix it by adding the selector.
Have two selectors to use the default of the FakeBTS.BTS code

fakebts/OpenBSCTest.st

@@ -116,6 +116,16 @@ Object subclass: OpenBSCTest [
         bts waitForBTSReady.
     ]
 
+    createAndConnectBTS: aNr [
+        <category: 'bts'>
+
+        bts := BTS new.
+        bts
+            btsId: aNr;
+            connect: 'localhost';
+            waitForBTSReady.
+    ]
+
     stopBts [
         <category: 'bts'>
         bts stop.