If the error message only triggered a shelltest warning instead of a
failure, then they're harmless, so we might as well silence them when
running 'redo test' (which runs t/shelltest.do, which enables
SHELLTEST_QUIET). We still want to print them when actually testing out
shelltest.od, though, to help with debugging the script.
This includes a fairly detailed test of various known shell bugs from the
autoconf docs.
The idea here is that if redo works on your system, you should be able to
rely on a *good* shell to run your .do files; you shouldn't have to work
around zillions of bugs like autoconf does.