Add test for ThrottledStream.Read method.
Add test for ThrottledStream.Read method.

This concerns issue #3787, which was fixed in pull request #3802.

mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,sitofabi/duplicati,sitofabi/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,sitofabi/duplicati,mnaiman/duplicati,sitofabi/duplicati,sitofabi/duplicati