Implement backward bisect

test/mod_list_troubleshooter_test.dart

@@ -97,5 +97,28 @@ void main() {
         expect(result.activeMods.length, equals(1));
       },
     );
+    test(
+      'Bisect search should end up with half the mods every backward iteration until 1',
+      () {
+        final troubleshooter = ModListTroubleshooter(modList);
+
+        var result = troubleshooter.binaryBackward();
+        // Half of our initial 30
+        expect(result.activeMods.length, equals(15));
+
+        result = troubleshooter.binaryBackward();
+        // Half of our previous result
+        expect(result.activeMods.length, equals(8));
+
+        result = troubleshooter.binaryBackward();
+        expect(result.activeMods.length, equals(4));
+
+        result = troubleshooter.binaryBackward();
+        expect(result.activeMods.length, equals(2));
+
+        result = troubleshooter.binaryBackward();
+        expect(result.activeMods.length, equals(1));
+      },
+    );
   });
 }