SLAPOS_TEST_LOG_DIRECTORY and SLAPOS_TEST_SHARED_PART_LIST have been
introduced recently, they are passed to runTestSuite using the usual
way of running runTestSuite once with --help to inspect supported
arguments and passing the argument only if runTestSuite supports them,
but these arguments are not necessarily heavy to compute and are always
known (unlike for example firefox path which is only known if seleniumrunner
extra software was installed), so simplify all this by passing the
parameters are environment variables.