tests/shell: don't redirect error/warning messages to stderr

tests/shell: don't redirect error/warning messages to stderr

Writing some messages to stderr and some to stdout is not helpful.
Once they are written to separate streams, it's hard to be sure about
their relative order.

Use grep to filter messages.

Also, next we will redirect the entire output also to a file. There the
output is also not split in two files.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>