selftest: Add helper function to create exports file for a testenv

selftest: Add helper function to create exports file for a testenv

This writes out the environment variables that are normally setup in the
testenv xterm to a file. This allows them to be sourced later.

This function is currently unused. However, it provides an alternative
replacement for the selftest-vars.sh script (which is really out of
date). I do plan to make use of this function in a subsequent patch-set.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>