test(cpu): add CPU micro tests for flags, DAA and stack ops

tests/GameBoy.Core.Tests/CpuMicroTests.cs

@@ -0,0 +1,53 @@
+using Xunit;
+
+namespace GameBoy.Tests
+{
+    public sealed class CpuMicroTests
+    {
+        private static Emulator Run(int instructions, params byte[] program)
+        {
+            var emulator = new Emulator(TestRoms.BuildProgram(program));
+            for (int i = 0; i < instructions; i++)
+                emulator.StepInstruction();
+            return emulator;
+        }
+
+        [Fact]
+        public void AddProducesZeroHalfAndCarryFlags()
+        {
+            var emulator = Run(3, 0x3E, 0x3A, 0x06, 0xC6, 0x80);
+            Assert.Equal(0x00, emulator.Cpu.A);
+            Assert.Equal(0xB0, emulator.Cpu.F);
+        }
+
+        [Fact]
+        public void PushThenPopMovesWordThroughStack()
+        {
+            var emulator = Run(4, 0x31, 0xFE, 0xFF, 0x01, 0x34, 0x12, 0xC5, 0xD1);
+            Assert.Equal(0x1234, emulator.Cpu.DE);
+        }
+
+        [Fact]
+        public void DaaConvertsBinaryAddToPackedBcd()
+        {
+            var emulator = Run(4, 0x3E, 0x45, 0x06, 0x38, 0x80, 0x27);
+            Assert.Equal(0x83, emulator.Cpu.A);
+        }
+
+        [Fact]
+        public void IncSetsHalfCarryOnNibbleRollover()
+        {
+            var emulator = Run(2, 0x3E, 0x0F, 0x3C);
+            Assert.Equal(0x10, emulator.Cpu.A);
+            Assert.True((emulator.Cpu.F & 0x20) != 0);
+            Assert.True((emulator.Cpu.F & 0x40) == 0);
+        }
+
+        [Fact]
+        public void SwapExchangesNibbles()
+        {
+            var emulator = Run(3, 0x3E, 0x3C, 0xCB, 0x37, 0x00);
+            Assert.Equal(0xC3, emulator.Cpu.A);
+        }
+    }
+}