setup_test_environment.sh: Depending on the operating system, or the
users installation of `vde_switch`, it might happen that the binary is not
located in the path even though the user has enough privileges to run it.

This issue has been observed in Fedora 18, by installing vde2 from source
with a suoder user.

This commit looks for the directory where `vde_switch` is located and adds
it to the PATH. This way, there is no need to add the requirement
"vde_switch must be in your PATH".