tests: stabilize test-fix.t for Windows

tests/test-fix.t

@@ -514,8 +514,11 b' print the filename if it is relevant.'
   $ printf "hello\n" > hello.txt
   $ hg add
   adding hello.txt
-  $ hg --config "fix.fail:command=printf 'HELLO\n' ; \
-  >                               printf '{rootpath}: some\nerror' >&2" \
+  $ cat >> $TESTTMP/cmd.sh <<'EOF'
+  > printf 'HELLO\n'
+  > printf "$@: some\nerror" >&2
+  > EOF
+  $ hg --config "fix.fail:command=sh $TESTTMP/cmd.sh {rootpath}" \
   >    --config "fix.fail:fileset=hello.txt" \
   >    fix --working-dir
   [wdir] fail: hello.txt: some