Clean up commented bits from QuantityInfo and IQuantityInfo (#1592)

- cleaned up some commented-out bits from `QuantityInfo` and
`IQuantityInfo`
- making the `IQuantityInfo` interface internal for `net8+`
- extended the test for
`XXX_QuantityInfo_ReturnsQuantityInfoDescribingQuantity` with the checks
for the explicit interface implementations
- simplified the test for `Ctor_SIUnitSystem_ReturnsQuantityWithSIUnits`

---------

Co-authored-by: Andreas Gullberg Larsen <andreas.larsen84@gmail.com>

Author: lipchev

CodeGen/Generators/UnitsNetGen/UnitTestBaseClassGenerator.cs

@@ -307,7 +307,7 @@ public virtual void Ctor_SIUnitSystem_ReturnsQuantityWithSIUnits()
         {{
             var quantity = new {_quantity.Name}(value: 1, unitSystem: UnitSystem.SI);
             Assert.Equal(1, quantity.Value);
-            Assert.True(quantity.QuantityInfo.UnitInfos.First(x => x.Value == quantity.Unit).BaseUnits.IsSubsetOf(UnitSystem.SI.BaseUnits));
+            Assert.True(quantity.QuantityInfo[quantity.Unit].BaseUnits.IsSubsetOf(UnitSystem.SI.BaseUnits));
         }}
 
         [Fact]
@@ -324,15 +324,19 @@ public void Ctor_UnitSystem_ThrowsArgumentExceptionIfNotSupported()
         [Fact]
         public void {_quantity.Name}_QuantityInfo_ReturnsQuantityInfoDescribingQuantity()
         {{
+            {_unitEnumName}[] unitsOrderedByName = EnumHelper.GetValues<{_unitEnumName}>().OrderBy(x => x.ToString()).ToArray();
             var quantity = new {_quantity.Name}(1, {_baseUnitFullName});
 
-            QuantityInfo<{_unitEnumName}> quantityInfo = quantity.QuantityInfo;
+            QuantityInfo<{_quantity.Name}, {_unitEnumName}> quantityInfo = quantity.QuantityInfo;
 
-            Assert.Equal({_quantity.Name}.Zero, quantityInfo.Zero);
             Assert.Equal(""{_quantity.Name}"", quantityInfo.Name);
-
-            var units = Enum.GetValues<{_unitEnumName}>().OrderBy(x => x.ToString()).ToArray();
-            var unitNames = units.Select(x => x.ToString());
+            Assert.Equal({_quantity.Name}.Zero, quantityInfo.Zero);
+            Assert.Equal({_quantity.Name}.BaseUnit, quantityInfo.BaseUnitInfo.Value);
+            Assert.Equal(unitsOrderedByName, quantityInfo.Units);
+            Assert.Equal(unitsOrderedByName, quantityInfo.UnitInfos.Select(x => x.Value));
+            Assert.Equal({_quantity.Name}.Info, quantityInfo);
+            Assert.Equal(quantityInfo, ((IQuantity)quantity).QuantityInfo);
+            Assert.Equal(quantityInfo, ((IQuantity<{_unitEnumName}>)quantity).QuantityInfo);
         }}
 
         [Fact]