If you can exclude some folder from Avast scanning, you can probably use Notepad to create a .txt file in that folder that contains the EICAR test file.
See the one-line test string listed under "Design":
(Don't include the  footnote symbol)
You need to create the file in a folder that Avast isn't scanning. Otherwise, Avast will "clean" the file right away.
Then, see if you can attach that file to two emails to yourself.
Open one when sandboxed and one when not sandboxed, to compare the two.