tests/shell: Fix flowtable test cases

tests/shell: Fix flowtable test cases

The major problem here was that existence of network interfaces 'eth0'
and 'wlan0' was assumed. Overcome this by just using 'lo' instead, which
exists even in newly created netns by default.

Another minor issue was false naming of 0004delete_after_add0 - the
expected return code is supposed to be separated by '_' from the
remaining filename.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>