Follow up to commit 12d00767f0
That commit moved the manifests into a subdirectory but failed to update the
relative path. That caused 'make check-system' to fail.
* etc/system-tests.scm (system-test-manifest): Set the source path to be the
root of the repository as it had been previously.
Change-Id: I68fb460861c4af080f5b14d1343d67b7e96109b0
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>