Specify the --build-config when invoking ctest.

Specify the --build-config when invoking ctest.

Otherwise we get strange build errors by attempting to build an
project in debug mode against a release-mode Qt:

http://testresults.qt-project.org/ci/QtBase_master_Integration/build_03955/win32-msvc2010_Windows_7/log.txt.gz

(grep for MSVCRTD)

The CMAKE_BUILD_TYPE variable is always specified by the ctest_testcase
driver as either Debug or Release.

Change-Id: Ia5af7f0c31550b92679a3190e07b4e5c4ea638c6
Reviewed-by: Brad King <brad.king@kitware.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>