Dart fix template tests (#4015)

* Clean up samples directory before fixing tests

- removed from samples/client/petstore/dart2 :
  - purge_test.sh (doesn't seem to be used and not helpful)
  - openapi folder (is to be re-generated with more meaningful name)
- updated dart2-petstore.sh to generate client library with new name
- used updated shell script to re-generate client library
- updated CI/.drone.yml to use the new client library for tests

* Update petstore tests to use faked http client

- skipped all of the tests that hit a live endpoint
- made a fake http client that can be set to check for expected values
   and/or return a provided response
- added some files with test data recorded from live api calls
- updated the README to reflect changes to tests

* Update .drone.yml so CI will run the tests

samples/client/petstore/dart2/openapi/git_push.sh

@@ -1,58 +0,0 @@
-#!/bin/sh
-# ref: https://help.github.com/articles/adding-an-existing-project-to-github-using-the-command-line/
-#
-# Usage example: /bin/sh ./git_push.sh wing328 openapi-pestore-perl "minor update" "gitlab.com"
-
-git_user_id=$1
-git_repo_id=$2
-release_note=$3
-git_host=$4
-
-if [ "$git_host" = "" ]; then
-    git_host="github.com"
-    echo "[INFO] No command line input provided. Set \$git_host to $git_host"
-fi
-
-if [ "$git_user_id" = "" ]; then
-    git_user_id="GIT_USER_ID"
-    echo "[INFO] No command line input provided. Set \$git_user_id to $git_user_id"
-fi
-
-if [ "$git_repo_id" = "" ]; then
-    git_repo_id="GIT_REPO_ID"
-    echo "[INFO] No command line input provided. Set \$git_repo_id to $git_repo_id"
-fi
-
-if [ "$release_note" = "" ]; then
-    release_note="Minor update"
-    echo "[INFO] No command line input provided. Set \$release_note to $release_note"
-fi
-
-# Initialize the local directory as a Git repository
-git init
-
-# Adds the files in the local repository and stages them for commit.
-git add .
-
-# Commits the tracked changes and prepares them to be pushed to a remote repository.
-git commit -m "$release_note"
-
-# Sets the new remote
-git_remote=`git remote`
-if [ "$git_remote" = "" ]; then # git remote not defined
-
-    if [ "$GIT_TOKEN" = "" ]; then
-        echo "[INFO] \$GIT_TOKEN (environment variable) is not set. Using the git credential in your environment."
-        git remote add origin https://${git_host}/${git_user_id}/${git_repo_id}.git
-    else
-        git remote add origin https://${git_user_id}:${GIT_TOKEN}@${git_host}/${git_user_id}/${git_repo_id}.git
-    fi
-
-fi
-
-git pull origin master
-
-# Pushes (Forces) the changes in the local repository up to the remote repository
-echo "Git pushing to https://${git_host}/${git_user_id}/${git_repo_id}.git"
-git push origin master 2>&1 | grep -v 'To https'
-