add: test for custom field auto register

tests/CustomFieldStructureTest.cs

@@ -0,0 +1,51 @@
+using System;
+using Skeleton.Algebra.FieldStructures;
+
+namespace Skeleton.Test;
+
+public class CustomFieldStructureTest
+{
+    [CustomFieldStructure]
+    public class CustomField : FieldStructure<int>
+    {
+        public override int Addition(int self, int other) => (self + other) % 7;
+        public override int AdditionUnit => 0;
+        public override int Multiplication(int self, int other) => (self * other) % 7;
+
+        public override int MultiplicationUnit => 1;
+        public override int AdditionInverse(int self) => (7 - self) % 7;
+
+        public override int MultiplicationInverse(int self)
+        {
+            for(int i = 0; i <= 7; i++ )
+                if ((i * self) % 7 == 1)
+                    return i;
+            throw new DivideByZeroException();
+        }
+
+        public override int Conj(int self) => self;
+
+        public override bool IsEqualApprox(int self, int other, double? absTol = null, double? relTol = null) =>
+            (self % 7) == (other % 7);
+
+        public override string RawCSharpRepresentation(int x) => $"{x}";
+
+        public override double MaxError(int a) => (Math.Abs(a) % 7);
+
+        public override int SquareRoot(int a)
+        {
+            for (int i = 0; i <= 7; i++)
+            {
+                if ((i * i) % 7 == a)
+                    return i;
+            }
+            throw new ArgumentException("negative number in sqrt");
+        }
+    }
+
+    [Test]
+    public static void CustomerFieldStructureShouldNotBeNullTest()
+    {
+        Assert.IsNotNull(FieldStructure<int>.Dispatch());
+    }
+}