nvs_flash: fixed deinit other partition's handles

* When deinitializing or erasing a partition,
  nvs used to close all handles instead of only
  the current partition's handles.
  This is fixed now
* Added a unit test for that case

Closes FCS-533

components/nvs_flash/src/nvs_handle_simple.cpp

@@ -130,4 +130,8 @@ bool NVSHandleSimple::nextEntry(nvs_opaque_iterator_t* it) {
     return mStoragePtr->nextEntry(it);
 }
 
+const char *NVSHandleSimple::get_partition_name() const {
+    return mStoragePtr->getPartName();
+}
+
 }