Move tests from nats package under test/* directory

[masumomo]

OVERVIEW

WHAT:

• As titled.

WHY:

• In order to prevent nats.go from bringing nats-server dependencies to go.mod.

DETAILS OF REQUEST

Reference

• This PR relates Move tests from nats package under test/* directory kubecon-contribfest#6

Not Implemented

• We have some tests that use internal fields, so I left them as it is for now. We will work on it in separated issues.
  • For example, nc.mu.Lock() and 'nc.mu.Unlock()' in tests.
  • I added comments // Need access to the internals for testing. for those tests.
• TestNoRaceObjectContextOpt sometimes fails, but it's unrelated to this change.
  • As I investigated, it might come from the timing when cancelling the context and shutdown the JetStream.

[wallyqs]

@masumomo Thanks for the contribution! 🙌 Could you revert the changes to go.mod and go.sum? You can instead use the following go_test.mod like this for running the tests to avoid including the server into the main go.mod:

go test ./... --count=1 -p=1 -modfile=go_test.mod

[masumomo]

Hello Wally! Oh, Thank you so much! I realised we have go_test.mod in the repo and an instruction on README to run the test 🙏 I'll revert it!

[piotrpio]

Hey @masumomo!

I checked out your branch, updated it and fixed remaining issues: #1441.
So I'll be closing this PR, but your contribution is merged into main (finally, sorry it took so long!).

Cheers, thanks for the help!