Fix configuration of LbExec tests

accidentally I noticed a couple of issues with the tests of LbExec:

• the exclusions workaround in pytest.ini works only if the build directory is inside the project directory
• pytest.ini is installed in InstallArea/python

After fixing the configuration of the test I realized that one of the tests creates a file in the "current directory", so I modified the test so that the file is created in a temporary directory although I believe the file should not be created at all (and the test should check that it is the case).