[tests] Run the tests as the current user, without needing root.

The sockets are placed in the _build directory rather than global
directories.
After an IPC error, wait a second before retrying.

This avoids a rapid spin loop that generally makes the operating
system and the user most unhappy.
Do not leave Sys.argv[0] blank in the executed tests.

This is to workaround ocsigen/lwt#23, since Lwt_log assumes that
`argv[0]` contains the executable name. This patch sets it to
the basename to avoid leaking the full path to the slave process
(which is presumably being chrooted shortly afterwards).