[go-experimental] Do not generate HTTP signature unit test for every generated SDK

Created by: sebastien-rosset

1. Move location of HTTP signature unit test file
2. Update go version to 1.14 in CI build
3. Add support for ed25519 signature

Previously I had a http_signature_test.mustache template that was used to generate Unit Tests for HTTP signature. The unit test was written for every generated client SDK. But in reality it's sufficient to have a unit test for one sample client. There is no reason to add the Unit Test to every generated client.

The unit test assumes there is a petstore API, which obviously does not exist in real SDKs. This ends up causing a compilation failure if the user calls "go test" in a generated SDK outside the sample petstore SDK.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.