bb.tests.fetch: Opt-out for unittests that require network

With this change, you can opt-out to skip unit tests that require an
internet connection. To opt-out, you'll set the environment variable
BB_SKIP_NETTESTS to 'yes'.

Signed-off-by: Olof Johansson <olof.johansson@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>