For the tests, we need to run commands in various contexts: in the host,
in a guest or in a namespace. Currently we do this by running each context
in a tmux pane, and using tmux commands to type the commands into the
relevant pane, then screen-scrape the output for the results if we need
them.
This is very fragile, because we have to make various assumptions to parse
the output. Those can break if a shell doesn't have the prompt we expect,
if the tmux pane is too small or in various other conditions.
This starts some library functions for a new "context" system, that
provides a common way to invoke commands in a given context, in a way that
properly preserves stdout, stderr and the process return code.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
