dtoc: Test the stdout output

dtoc: Test the stdout output

Normally dtoc outputs to a file but it also offers a way to write output
to stdout. At present the test for that does not actually check that the
output is correct. Add this to the test.

This uses a member variable to hold the expected text, so it can be used
in muitiple places.

Signed-off-by: Simon Glass <sjg@chromium.org>