Reorganize tests by API and service concerns

RpgRoller.Tests/Services/RandomDiceRollerTests.cs

@@ -0,0 +1,17 @@
+using RpgRoller.Services;
+
+namespace RpgRoller.Tests;
+
+public sealed class RandomDiceRollerTests
+{
+    [Fact]
+    public void RandomDiceRoller_ProducesValueWithinRange()
+    {
+        var roller = new RandomDiceRoller();
+        for (var i = 0; i < 64; i += 1)
+        {
+            var value = roller.Roll(12);
+            Assert.InRange(value, 1, 12);
+        }
+    }
+}