[Golang][client] update test dependencies

Created by: grokify

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
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@antihax @bvwells

Description of the PR

Update tests to use context instead of golang.org/x/net/context. context is a core lib as of Go 1.7 and generated code uses context.

https://godoc.org/golang.org/x/net/context

package context import "golang.org/x/net/context"

Package context defines the Context type, which carries deadlines, cancelation signals, and other request-scoped values across API boundaries and between processes. As of Go 1.7 this package is available in the standard library under the name context. https://golang.org/pkg/context.

Also reordered so dependencies so they are consistent across files. Three groups are used:

1. core libs
2. other dependencies
3. petstore client