tst_QString: extend arg() tests with enums w/o explicit underlying_type

The QtDeclarative code causing QTBUG-131906 hits UB, because it tries
to store -666 in an enum {A, B}, which has a valid range of [0,1],
therefore its underlying_type is uint, yet, as per [conv.prom]/3¹,
integer-promotes to _int_ instead, so in Qt 6.8 would cause the
arg(int) overload to be called, outputting -666, while in Qt 6.9, it's
treated (correctly) as an unsigned value, outputting -666's two's
complement, a positive value.

Add a version of the scenario that does not cause UB.

¹ Thanks to Ahmad Samir for digging up the pertinent legalese.

Task-number: QTBUG-131906
Pick-to: 6.9 6.8 6.5
Change-Id: Iba1a04de523a0b4cd1c87deea40c643cf16df14f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

tests/auto/corelib/text/qstring/tst_qstring.cpp

@@ -6695,6 +6695,11 @@ void tst_QString::arg()
              QLatin1String("[9223372036854775808]") );
 
     // (unscoped) enums
+    enum Enum {
+        Foo1, Foo2,           // reproducer for QTBUG-131906,
+        RangeExtended = -667, // but w/o the UB of out-of-range values
+    };
+    QCOMPARE(s4.arg(Enum(-666)), QLatin1String("[-666]"));
     enum : int { FooS = -1 };
     enum : uint { FooU = 1 };
     QCOMPARE(s4.arg(FooS), QLatin1String("[-1]"));