QMake doesn't use specified spec when running config tests

When running tests with qtCompileTest(), those tests are always run with the default spec, not with the spec the user explicitly passed to qmake.

In my case, I have a config test that tests for certain C++14 features, and invoke qmake with "qmake -spec linux-clang". However, the tests are run with the spec Qt was built with, which is linux-g++. Therefore, the tests will fail, even though the C++14 features I wanted to test for are available with linux-clang.