dtc: Testcases for input handling

This patch adds some testcases checking corner cases of dtc's input
file handling.  Specifically it checks that dtc works correctly when
given input via stdin, and it checks that dtc fails gracefully if
given a nonexistent input file (or directory, in the case of -Ifs
mode).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>