selftest: net: Fix error message if empty variable

selftest: net: Fix error message if empty variable

Fix to avoid cases where the `res` shell variable is
empty in script comparisons.
The comparison has been modified into string comparison to
handle other possible values the variable could assume.

The issue can be reproduced with the command:
make kselftest TARGETS=net

It solves the error:
./tfo_passive.sh: line 98: [: -eq: unary operator expected

Signed-off-by: Alessandro Zanni <alessandro.zanni87@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250925132832.9828-1-alessandro.zanni87@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>