Add a new 'CT_FLAG_STDOUT_STRING' option to /bin/cheritest test cases,
which allows a string to be specified that is expected to appear on the
test process's stdout.  Implement this for the 'invoke_helloworld' test;
we will also want to implement it for other test cases, especially those
relating to file-descriptor objects.
Use a sligthly shorter and more deterministic definition of 'hello world'
in the CHERI system object that makes for a better test case with
/bin/cheritest.