[AI Test Tool] Fix GetNextTurn (#3324)

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#### Summary <!-- Provide a general summary of your changes -->
Causes indexing error in multi-turn testing. 

Current turn should never be larger than number of turns. Otherwise
GetTestInput will fail:
`TestInputJson := TestInput.GetTestInput(TurnsTok).ElementAt(CurrentTurn
- 1).Element(ElementName)`

#### Work Item(s) <!-- Add the issue number here after the #. The issue
needs to be open and approved. Submitting PRs with no linked issues or
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Fixes
[AB#571847](https://dynamicssmb2.visualstudio.com/Dynamics%20SMB/_workitems/edit/571847)

Author: stkillen

src/Tools/AI Test Toolkit/src/AITTestContextImpl.Codeunit.al

@@ -186,7 +186,7 @@ codeunit 149043 "AIT Test Context Impl."
         if not IsMultiTurn then
             exit(false);
 
-        if CurrentTurn > NumberOfTurns then
+        if CurrentTurn + 1 > NumberOfTurns then
             exit(false);
 
         CurrentTurn := CurrentTurn + 1;