t/shelltest.od: quiet some error messages that get printed during testing.

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.

t/shelltest.do

@@ -1,4 +1,5 @@
 set +e
+export SHELLTEST_QUIET=1
 ( . ./shelltest.od )
 RV=$?
 case $RV in