test_activesync_general green again, imap parallel orange
minor changes made during exploration of bug 896151
progress on test_compose for activesync

Have devicestorage clean up after itself since test_compose likes to save
files to devicestorage using their default name, and that will result in
collisions for the second test to run.  I tried to have us give each test
and variant its own devicestorage path, but that is totally ineffective
since we can't control the sdcard path that gets used, at least on b2g18.
We might want to file an enhancement bug for that...
test_compose is green for imap and activesync
perma-green test_imap_parallelfetch

I had broken this when playing with it before when I only half-removed use of
JSON.stringify.  But this also fixes bug 896151 that I filed.