Skip incremental scan in residue mode

PerfectNumbers.Core.Tests/MersenneNumberTester/MersenneResidueModeTests.cs

@@ -12,5 +12,12 @@ public class MersenneResidueModeTests
         var tester = new MersenneNumberTester(useResidue: true, useIncremental: false);
         tester.IsMersennePrime(136_000_002UL).Should().BeFalse();
     }
+
+    [Fact]
+    public void IsMersennePrime_residue_mode_accepts_prime_exponent()
+    {
+        var tester = new MersenneNumberTester(useResidue: true, useIncremental: true, maxK: 1_000UL);
+        tester.IsMersennePrime(31UL).Should().BeTrue();
+    }
 }