tests/stdio: normalize whitespace

space before star

Make opts.cmd const char * rather than char * -- nothing ever modifies
the string it points to, it is never passed to a function not taking a
const char * arg, and this allows "" (or other literal strings if ever
needed) to be assigned to it - which should fix the build.

fixes from RVP

fix printf format strings

Add interrupted I/O tests (from RVP)