ua: Add a testcase for M2UAMSG>>#findTag:ifAbsent:, fix the code

Make the findTag:ifAbent: work. The tag didn't have the isTag
query selector.

Tests.st

@@ -308,6 +308,18 @@ TestCase subclass: M2UAMSGTests [
         self assert: msg msgClass = UAConstants clsMAUP.
         self assert: msg msgType = UAConstants maupData.
     ]
+
+    testFindTag [
+        | inp msg tag |
+        inp := #(16r01 16r00 16r03 16r01 16r00 16r00 16r00 16r10 16r00
+                    16r11 16r00 16r08 16rAC 16r10 16r01 16r51) asByteArray.
+        msg := M2UAMSG parseFrom: inp.
+        tag := msg findTag: M2UAConstants tagReserved ifAbsent: [ nil ].
+        self assert: tag isNil.
+
+        tag := msg findTag: M2UAConstants tagAspIdent ifAbsent: [self fail].
+        self deny: tag isNil.
+    ]
 ]
 
 TestCase subclass: OsmoUDPSocketTest [

ua/M2UA.st

@@ -282,6 +282,11 @@ Object subclass: M2UATag [
             aMsg putByteArray: (ByteArray new: 4 - rest).
         ].
     ]
+
+    isTag: aNr [
+        <category: 'accessing'>
+        ^ self nr = aNr
+    ]
 ]
 
 Object subclass: M2UAMSG [