Add http request mocking support

[swissspidy]

Fixes #210

• Depends on Add hook to http_request() util wp-cli#6036 for mocking requests done by the WP-CLI http_request() util

[swissspidy]

@BrianHenryIE Would love to hear your thoughts on this especially :-)

Note:The original suggestion was to store the mocked responses in files and use those, but this can be achieved with this solution just as well.

Example:

  Background:
    When I run `cat my/mocked/response.json`
    Then save STDOUT as {HTTP_RESPONSE}

  Scenario: Mock HTTP request in WP-CLI
    Given an empty directory
    And an HTTP request to https://api.github.com/repos/wp-cli/wp-cli/releases?per_page=100 with this response:
    """
    HTTP/1.1 200
    Content-Type: application/json

    {HTTP_RESPONSE}
    """

I haven't tested that though, but I could add a test for it.

[swissspidy]

By the way this has already been successfully tested as part of wp-cli/wp-cli#6037, so I think it's good to go.

Appreciate a good review though :)