Due to a series of mistakes, trying to change a message with an onBeforeSend listener doesn't actually do anything. This needs proper automated tests.

The actual mistakes are fixed by this bit:

-          setComposeState(composeWindow, result.details);
+          await setComposeDetails(composeWindow, result.details);
+          composeWindow.GetComposeDetails();

The first mistake was forgetting to await setComposeDetails. The second was not actually updating the details for the send function (which we interrupted to get here).

Also in this patch I've renamed setComposeState to setComposeDetails which I'd intended to do before but missed.

And I've beefed up the test. It now actually "sends" messages (send later), then reads the sent message from the outbox.

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/75aa7d5dbcfd
Make compose.onBeforeSend listener actually change composed message. r=mkmelin DONTBUILD

Thunderbird 74.0b2:
https://hg.mozilla.org/releases/comm-beta/rev/8c986f1b128d