PY-17017 Add a test on actual custom folding of such trailing blocks

python/testData/folding/customFoldingAtBlockEnd.py

@@ -0,0 +1,5 @@
+class C:<fold text='...'>
+    <fold text='Custom'># region Custom
+    def m(self):<fold text='...'>
+        pass</fold>
+    # endregion</fold></fold>

python/testSrc/com/jetbrains/python/PyFoldingTest.java

@@ -66,4 +66,8 @@ public class PyFoldingTest extends PyTestCase {
     doTest();
   }
 
+  // PY-17017
+  public void testCustomFoldingAtBlockEnd() {
+    doTest();
+  }
 }