tests: Make read_mail overwrite project correctly

Mail object append headers, so to overwrite them correctly we have to
delete the original one first.

Also fix a chicken and egg problem spotted by this fix.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
1 job from master in 9 minutes and 18 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #58597
test

00:09:18