Not all our images are able to run the tests. Rather than use features
we can just check for the existence and run-ability of gtester. If the
image has been setup for binfmt_misc it will be able to run anyway.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>