selftests..multiplex_tests: Check that vars changes

Replace test_run_mplex_timeout with test_run_mplex_params test which
executes 'env_variables.sh' and checks that output contain correct
output. This test checks the multiplexer capability to change variable
value.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #293 from ldoktor/selftest-mux

selftests..multiplex_tests: Check that vars changes