ssconf: Verify file size when reading, add some tests

Until now ssconf would limit the amount read from files to 128 KiB and
silently ignored files larger than that. With this patch a check is
added by using fstat(2) on the file descriptor while it's being read.

There were no tests file ssconf at all, so some are added.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>