Fix qbs project files for valgrind autotests.

They did not build on Windows.

Change-Id: I0c688b8e1b78e9bb06e2efd6e2cbf8791f272bc2
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Autotests: Do not set the QT_USE_FAST_CONCATENATION macro in qbs build.

The one test for which we included it does not need it anymore,
and the StringUtils test fails to link on Windows when it is present,
so remove it again.

Change-Id: Iebe810c82842047a323f65722a61940271f921f3
Reviewed-by: Orgad Shaneh <orgads@gmail.com>