Support building tests with an installed Qt which was built with cmake

Currently tests can only be built as part of the initial qtbase build stage.

This is a problem, because Coin expects to build qtbase, install it, package it, and then in a separate Coin step, extract the installed qtbase package, and build tests against the unpackaged (installed) qtbase.

For this to work we would have to do a couple of things:
1) Install any required cmake modules (like QtBuild, which is actually currently installed, but some other ones are not).

2) Modify those modules to allow building tests as part of a separate cmake invocation against an installed qt (so do necessary changes in add_qt_test).

3) Modify the root tests CMakeLists.txt file to support building tests both inside the initial qtbase built tree, or against an installed qtbase.

This is slightly related to QTBUG-74713.