MinGW is very sensitive to inlining / export errors, so having a
developer build check is valuable. Add a MinGW developer build
executing tests to increase coverage and turn off testing for the
packaging build in return.

Change-Id: Ia4bdacc9952c7a568ac9d4105818eb926c88e6ca
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>