Add workflow to test Elixir clients

[wing328]

• add github workflow to test Elixir client
• revise existing tests

FYI @mrmstn (2018/12)

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

@efcasado can you please review the test failure when you've time?

I think we may need to update the tests after merging some of your PRs (e.g. update dependencies to newer versions)

[efcasado]

I'll do!

I wasn't aware the generator was generating any tests (didn't bother checking either 😓). At first glance, it seems the issue was introduced by #21039 which is when Jason was retired in favor of the built-in JSON module.

Do you want me to create a separate branch for the fixes, or do you want me to contribute directly to this pull request?

[wing328]

do you want me to contribute directly to this pull request?

directly to this branch please

[efcasado]

I cannot push to this branch directly, I will have to do the changes in my fork an open a pull request towards this branch (maybe this is what you meant, just want to be clear about expectations). Either way, a fix will be available soon 🙏

[wing328]

@efcasado fyi. i updated the spec to use localhost and start a petstore server locally.

all elixir tests passed.

thanks for updating the tests to make all the tests passed