[1.5.x] Fixed #19645 -- Added tests for TransactionMiddleware

Backpatch of f556df90be995a83b979cf875705d98521ab4dc7. Backpatching
these tests so that it will be easier to backpatch the fix for #19707.
[1.5.x] Fixed #19707 -- Reset transaction state after requests

Backpatch of a4e97cf315142e61bb4bc3ed8259b95d8586d09c