testing: Add a script to build the current (or an arbitrary) source tree

WIP: It's unfortunate that the source dir is modified, if we'd limit us to
Git source dirs we could quickly build a tarball via Git and extract that in
the builddir (overwriting existing files to allow incremental builds)
then run autoreconf as we do now.

This allows to (relatively) quickly (re-)build and install the current
or an arbitrary strongSwan source tree within the root image.

As with building the root image in general the guests must not be
running while executing this script.  The guests' images do not have to
be rebuilt, though, as these are automatically rebased to the new root so
no configuration files or other modifications are lost.
WIP: Use Git to create a temp stash and an archive from it to create a separate source tree

The problem here is that `git stash create` has no --include-untracked
option, so only modified files are included not new ones that have not
yet been added to the index.