When SHELL is unset the test fails with

150,151c150,151
< ./redir11.sub: line 26: echo: write error: Bad file number
< ./redir11.sub: line 27: echo: write error: Bad file number
---
> ./redir11.sub: line 26: $(a=4 foo): Bad file number
> ./redir11.sub: line 27: $(a=4 foo): Bad file number

--- tests/redir.tests	2016-12-01 11:20:04.910983199 +0000
+++ tests/redir.tests	2016-12-01 11:19:34.114069067 +0000
@@ -1,7 +1,7 @@
 : ${TMPDIR:=/tmp}
 
 export LC_ALL=C
-export LANG=C
+export LANG=C; export SHELL=
 
 # catch-all for remaining untested redirection stuff
 set +o posix