Use testing.TempDir() where possible

[mprimi]

Rather than relying on manual deferred cleanup of files, switch most tests to use testing.TempDir() which is automatically cleaned up.

• [ ] Link to issue, e.g. Resolves #NNN
• [ ] Documentation added (if applicable)
• [x] Tests added
• [x] Branch rebased on top of current main (git pull --rebase origin main)
• [x] Changes squashed to a single commit (described here)
• [ ] Build is green in Travis CI
• [x] You have certified that the contribution is your original work and that you license the work to the project under the Apache 2 license

Resolves #

Changes proposed in this pull request:

• Change test utility methods to use testing.TempDir()
• Remove (now superfluous) deferred directory and file cleanup

/cc @nats-io/core

[mprimi]

@kozlovic holding until after 2.9 sounds good (PR is still marked draft, I only opened to see if Travis would surface any error I wasn't seeing locally (it did, fixed)