[OCaml][test] Use correct data layout string. (#68781)

core.ml would previously set a data layout string of "e" and check that
it remained "e". This is fragile when we have the data layout string
auto-upgrade facility, and indeed broke when D86310 upgraded this one.
As the auto-upgrade logic is not what is being tested here, it seems
easier to just use the data layout string that the target expects.