[FIXED] Get rid of server dev dependencies in nats package

[piotrpio]

• When possible, tests were moved under test directory
• Some tests were refactored/removed to not use unexported properties
• Introduced -tags=internal_testing for tests that depend on internals not exposed via public API

[wallyqs]

Confirmed this approach would work as below ✅

$ echo '
package main

import (
	"github.com/nats-io/nats.go"
)

func main() {
	// Connect to a server
	nc, _ := nats.Connect(nats.DefaultURL)

	// Simple Publisher
	nc.Publish("foo", []byte("Hello World"))
}
' > main.go

$ go mod init example.com/example
$ go mod edit -require github.com/nats-io/nats.go@move-server-tests
$ go mod tidy
go: downloading github.com/nats-io/nats.go v1.30.3-0.20231013231123-4065e270471b

$ cat go.mod

module example.com/example

go 1.21.2

require github.com/nats-io/nats.go v1.30.3-0.20231013231123-4065e270471b

require (
	github.com/klauspost/compress v1.17.0 // indirect
	github.com/nats-io/nkeys v0.4.5 // indirect
	github.com/nats-io/nuid v1.0.1 // indirect
	golang.org/x/crypto v0.6.0 // indirect
	golang.org/x/sys v0.5.0 // indirect
)

$ go build .
$ go version -m example
example: go1.21.2
	path	example.com/example
	mod	example.com/example	(devel)	
	dep	github.com/klauspost/compress	v1.17.0	h1:Rnbp4K9EjcDuVuHtd0dgA4qNuv9yKDYKK1ulpJwgrqM=
	dep	github.com/nats-io/nats.go	v1.30.3-0.20231013231123-4065e270471b	h1:gFa8SUr5qFCHvtpGoDMvGwzs106hBZiDWo37JO7EaHU=
	dep	github.com/nats-io/nkeys	v0.4.5	h1:Zdz2BUlFm4fJlierwvGK+yl20IAKUm7eV6AAZXEhkPk=
	dep	github.com/nats-io/nuid	v1.0.1	h1:5iA8DT8V7q8WK2EScv2padNa/rTESc1KdnPw4TC2paw=
	dep	golang.org/x/crypto	v0.6.0	h1:qfktjS5LUO+fFKeJXZ+ikTRijMmljikvG68fpMMruSc=
	dep	golang.org/x/sys	v0.5.0	h1:MUK/U/4lj1t1oPg0HfuXDN/Z1wv31ZJ/YcPiGccS4DU=
	build	-buildmode=exe
	build	-compiler=gc
	build	CGO_ENABLED=1
	build	CGO_CFLAGS=
	build	CGO_CPPFLAGS=
	build	CGO_CXXFLAGS=
	build	CGO_LDFLAGS=
	build	GOARCH=amd64
	build	GOOS=darwin
	build	GOAMD64=v1
	build	vcs=git
	build	vcs.revision=960f37f30fc9728329b75b5a937d1714c94e1ad3
	build	vcs.time=2023-10-13T23:12:01Z
	build	vcs.modified=true

[wallyqs]

LGTM, when it is ready and CI green.

[AlexCuse]

This is awesome thanks so much 🫡