net-test: rootfs/net_test.sh - minor stylistic fixes

Google shell style guide: https://google.github.io/styleguide/shell.xml


says to always use "${x}" instead of $x, and in general to double quote
everything (fixes issues with whitespace in variable expansions).

Similarly [[ is bash builtin, unlike [ or test.

Bug: 126465735
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I507ceb630e7dd7347783e499826dcce2c2c50ee5