FIX: Add UNITY_INPUTSYSTEM_SUPPORTS_MOUSE_SCRIPT_EVENTS to tests asmdef so MouseEvents tests compile

versionDefines are scoped to the asmdef that declares them and do not propagate to
referencing assemblies. UNITY_INPUTSYSTEM_SUPPORTS_MOUSE_SCRIPT_EVENTS was declared
only on the runtime asmdef (Unity.InputSystem.asmdef, Unity >= 6000.4.0a4), so the
symbol was always undefined in the test assembly. The entire CoreTests_MouseEvents.cs
file is gated by #if UNITY_INPUTSYSTEM_SUPPORTS_MOUSE_SCRIPT_EVENTS, which meant the
MouseEvents_CanReceive* tests (including the regression test that catches the
OnMouseUpAsButton focus-update bug fixed in the previous commit) silently compiled
out and never ran in CI. Mirror the runtime versionDefine on Unity.InputSystem.Tests.asmdef.

Author: LeoUnity

Assets/Tests/InputSystem/Unity.InputSystem.Tests.asmdef

@@ -72,6 +72,11 @@
             "expression": "6000.3.0a6",
             "define": "UNITY_INPUT_SYSTEM_PLATFORM_POLLING_FREQUENCY"
         },
+        {
+            "name": "Unity",
+            "expression": "6000.4.0a4",
+            "define": "UNITY_INPUTSYSTEM_SUPPORTS_MOUSE_SCRIPT_EVENTS"
+        },
         {
             "name": "Unity",
             "expression": "6000.5.0a8",