Add documentation on how to write tests for telescope.nvim

[ColinKennedy]

Is your feature request related to a problem? Please describe.
I'm writing an extension and, despite checking every extension from the wiki, I couldn't find any that actually runs the their code via telescope calls. The best I saw was a couple plugins test their own internal logic and don't even attempt to call their code the way a user might.

Describe the solution you'd like
Be able to call a picker and query its contents in a unittest / integration test.

  it("shows up with 1 saved session", function()
    -- Some prologue code that could affect `assert.equal` later

    local picker = viewer.create()
    picker:find()
    assert.equal(1, picker.manager:num_results())  -- Some meaningful check
  end)

Describe alternatives you've considered
n/a

[jamestrew]

Probably the closest thing to this would be these tests https://github.com/nvim-telescope/telescope.nvim/blob/master/lua/tests/automated/pickers/find_files_spec.lua

But even in our code base, its usage is very limited and hasn't been used for anything but some common operations for the find_files picker.
I tried writing some integration tests for another extension previously but I found it quite tricky (for my usecase). I would like to improve this aspect but I'm constrained for time.