Improve test coverage for TypeUtils.isAssignable (#1574)

* [LANG-1812] Test: Expose and test TypeUtils.isAssignable to improve coverage

* Refactor: Revert method visibility to private and test via public API

* Fix: Checkstyle violations (final keywords and whitespace)

* Clean up tests

---------

Co-authored-by: Dhaneesh <dhaneesh646@gmail.com@gmail.com>
Co-authored-by: Gary Gregory <garydgregory@users.noreply.github.com>

Author: 3 people

src/test/java/org/apache/commons/lang3/reflect/TypeUtilsTest.java

@@ -1226,4 +1226,24 @@ void testWrap() {
         assertTrue(TypeUtils.equals(t, TypeUtils.wrap(t).getType()));
         assertEquals(String.class, TypeUtils.wrap(String.class).getType());
     }
+
+    @Test
+    public void testIsAssignable_Type_TypeVariable_Map_Public() throws Exception {
+        class Container<T> {
+        }
+        final TypeVariable<?> typeVarT = Container.class.getTypeParameters()[0];
+        final boolean result = TypeUtils.isAssignable(Integer.class, typeVarT);
+        assertFalse(result);
+    }
+
+    @Test
+    public void testIsAssignable_HiddenMapLogic() {
+        class Container<T> {
+        }
+        class StringContainer extends Container<String> {
+        }
+        final Type stringContainerType = StringContainer.class.getGenericSuperclass();
+        assertTrue(TypeUtils.isAssignable(StringContainer.class, Container.class));
+    }
+
 }