self-test : use python modules from build, various fixes

Various problems with self-tests of pyside2 (ongoing).

1. `BUILD_DIR` env var is wrong: must be `CMAKE_BINARY_DIR` .
2. build modules must be pointed by their parent directories
3. fix python warning : `is not` should be `!=`
4. tests fails because of the QPA backend:
   with QT_QPA_PLATFORM=offscreen, all OpenGL and Qml tests fails
   with QT_QPA_PLATFORM=vnc, OpenGL tests are KO but Qml tests are fine

Context: Qt 5.15.1, python 3.9, musl libc, VoidLinux distribution

Without patches, 0% of tests passed.
With the proposed patches: "96% tests passed, 21 tests failed out of 471"