feat(tests): add range supports

[lowlighter]

May close #263 and close #288

This add support for the range attribute that can be passed in a step context.
The following types along their contexted variables are supported:

• []interface{}: an array of values
  • .index/.key: current iteration index
  • .value: current iteration item value
• map[string]interface{}: a map of values (each key will be iterated over)
  • .index: current iteration index
  • .key: current iteration item key
  • .value: current iteration item value
• int: an integer to perform action n times
  • .index/.key/.value: current iteration index
• string: a templated string which results in one of the above typing
  • It can be either inherited from vars file, or interpolated from a previous step result

Within a range context, .range variable will be set to true (while it'll be false in "regular" contexts)

Here's an example (see tests/ranged.yml in this PR diff for more examples):

- name: range with harcoded array
  steps:
  - type: exec
    range:
      - actual: hello
        expected: hello
        index: 0
      - actual: world
        expected: world
        index: 1
    script: echo "{{.range}} {{.index}} {{.value.actual}}"
    assertions:
    - result.code ShouldEqual 0
    - result.systemout ShouldEqual "true {{.value.index}} {{.value.expected}}"

Image below show the content of Range struct for each test case:

---

I haven't run go fmt yet as it'll enclose the current step processing (L176 to L281) within the for which make the diff harder to review. If no further changes are requested, I'll run the formatter and add some documentation in the README.md with definitive syntax 🙂

[fsamin]

Thanks for the PR.
Here are some feedback.

Can you also add a new section in the README ?

[lowlighter]

• Applied feedback changes and rebased to fix conflict with feat(executors): expose previous steps result #452
• Updated the README.md to add section about range expression
• Also applied go fmt (which makes the diff harder to read because it changes the indents, can revert it temporarly for review if you want)