binman: Avoid use of expected failure

The testReplaceSectionSimple() test is the only one which expects failure.
It looks odd in the output and takes time to glance at it to see that all
is in fact well. Also it does not check that the right exception is
generated.

Use the more common (in binman) approach of checking for an exception.

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