FlashDevelop now seems to have complete support for debugging workers which really wasn't the case on older versions (you could neither break, or trace inside workers). AIR SDK workers support has also progressed (I remember things working in release would break in debug version) I just recompiled your sample with AIR SDK 14 release (14.0.0.110)
air14_sdk_win/bin/mxmlc -swf-version=25 -debug=true Main.as
and debugged it with Shockwave Flash Debugger 14,0,0,125 and FlashDevelop 4.6.1.30 and got the expected result:
[Starting debug session with FDB]
Created Worker 2
test
Beware that any element not up-to-date in your debugging chain (sdk/player/debugger) could result in problems for debugging workers