Fix wrong variable name

This commit should come with a test, but I'm offline and you can't
generate this error while offline.
add "scoped" Sandbox.cd to tests

I honestly have found it difficult in the old cli-test.sh to figure out
which tests were inside which directories and wanted something like
this there.
self-test: detect online status

rationale: I had no idea that "self-test --offline" existed, and I even
gave code review suggesting that it be implemented! Since we have code
already to detect whether you are online, and we print a clear message
about the skipped tests, make the default convenient.

Automated QA (which want to always run every test) can pass
--force-online instead.
Add 'net' tag to some 'slow' tags that use npm

'meteor self-test --slow' now passes while offline
grammar: make 'net' skip message match others

I am resisting writing the code to change the grammar if only 1 test is
skipped due to that tag.
Update update comment

Unless I'm confused, the code in the comment also runs in the pretty
typical case where you already have the latest release on your system
but the app you're in is pinned to an older release, and you run "meteor
update" without passing --release.  And that case was not described by
the previous comment.
Fix "meteor update --release some-old-release"
Don't describe update --release as "latest"

This bug predated tool-refactoring.