Rewrite some legacy tests

.editorconfig

@@ -51,8 +51,8 @@ dotnet_diagnostic.FAA0004.severity = error  # Replace NUnit assertion with Fluen
 dotnet_diagnostic.NUnit1001.severity = none       # The individual arguments provided by a TestCaseAttribute must match the type of the corresponding parameter of the method
 dotnet_diagnostic.NUnit2005.severity = suggestion # Consider using Assert.That(actual, Is.EqualTo(expected)) instead of Assert.AreEqual(expected, actual)
 dotnet_diagnostic.NUnit2021.severity = warning    # Incompatible types for EqualTo constraint
-
-
+dotnet_diagnostic.NUnit2005.severity = error      # Consider using Assert.That instead of AreEqual
+dotnet_diagnostic.NUnit2033.severity = error      # Consider using Assert.That instead of NotZero
 
 dotnet_diagnostic.QW0001.severity = none    # Use a testable Time Provider
 dotnet_diagnostic.QW0010.severity = none    # Use DateOnly instead of Date
@@ -113,7 +113,6 @@ dotnet_diagnostic.S3925.severity = none # "ISerializable" should be implemented
 dotnet_diagnostic.S4136.severity = none # Method overloads should be grouped together
 
 dotnet_diagnostic.S1133.severity = suggestion # Deprecated code should be removed
-dotnet_diagnostic.S3900.severity = suggestion # Arguments of public methods should be validated against null
 
 dotnet_diagnostic.S107.severity = warning  # Methods should not have too many parameters
 dotnet_diagnostic.S127.severity = warning  # Methods should not have too many parameters
@@ -136,6 +135,7 @@ dotnet_diagnostic.S3215.severity = warning # Interface instances should not be c
 dotnet_diagnostic.S3218.severity = warning # Inner class members should not shadow outer class "static" or type members
 dotnet_diagnostic.S3257.severity = warning # Declarations and initializations should be as concise as possible
 dotnet_diagnostic.S3776.severity = warning # Cognitive Complexity of methods should not be too high
+dotnet_diagnostic.S3900.severity = warning # Arguments of public methods should be validated against null
 dotnet_diagnostic.S6354.severity = warning # Use a testable (date) time provider instead
 
 dotnet_diagnostic.VSSpell001.severity = silent # Correct spelling

specs/Qowaiv.Specs/Date_span_specs.cs

@@ -1,4 +1,6 @@
-namespace Date_span_specs;
+using FluentAssertions.Extensions;
+
+namespace Date_span_specs;
 
 public class Is_comparable
 {
@@ -141,6 +143,16 @@ public void plus_does_nothing()
     }
 }
 
+public class Can_be_added_to
+{
+    [Test]
+    public void Date_times()
+    {
+        var date = 30.January(1999).At(13, 42).AsUtc();
+        date.Add(DateSpan.FromMonths(1)).Should().Be(28.February(1999).At(13, 42).AsUtc());
+    }
+}
+
 public class Can_create
 {
     [Test]

specs/Qowaiv.Specs/Debug_SVO_specs.cs

@@ -42,7 +42,7 @@ public void display(Type svoType, object display)
 public class Unknown_value : SingleValueObjectSpecs
 {
     public static IEnumerable<Type> SvosWithDefaultUnknown
-        => SvosWithUnknown.Except(new[] { typeof(Sex), typeof(InternetMediaType) });
+        => SvosWithUnknown.Except([typeof(Sex), typeof(InternetMediaType)]);
 
     [TestCaseSource(nameof(SvosWithDefaultUnknown))]
     public void displays_unknown_for(Type svoType)