trigger-popup: trigger node should have a themeable class

**Bug**

The trigger node of `trigger-popup` should have a themeable class applied to it:
https://github.com/dojo/widgets/blob/a3fdfc3216aec67fb4cdb794906379b91c7796bb/src/trigger-popup/index.tsx#L59

Also does `classes` have any effect on `virtual`?
https://github.com/dojo/widgets/blob/a3fdfc3216aec67fb4cdb794906379b91c7796bb/src/trigger-popup/index.tsx#L58

Package Version: 7.0.3, 8.0.0

Start at src/trigger-popup/index.tsx around lines 58-59, as linked in the issue, and inspect how the trigger node and virtual element receive classes. Confirm whether classes affects virtual, then verify that the trigger node has a themeable class and add or update the relevant tests if the repository provides them.

Written by the indexing model from the issue text.