tests: explicitly select to open an attachment in Thunderbird
tests: explicitly select to open an attachment in Thunderbird

The default ("open" or "save") for attachment may not be always the same
- Thunderbird somehow guesses what user wants to do. Since the test is
about "open" action, select it explicitly, instead of trying to convince
TB to have it by default.

QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin