@Tyagov, please review.

Now the test runners use the LD_PRELOAD environment variable to load
the correct libstdc++ built by slapos and restart themselves.

The restart is necessary because LD_PRELOAD is only taken into account
at the time the process is started. Changing this environment variable
in runtime has no effects.

Tests results are available at Nexedi's ERP5 and are 100% ok: https://nexedi.erp5.net/test_result_module/20160927-3FA05624.

/reviewed-on !99