Allow adding new mocks in Tesla.Mock.mock instead of overwriting previously defined mocks

[Nezteb]

Note: most of this wouldn't be an issue depending on the results of #241

Say I have a test:

def setup do
  Tesla.Mock.mock(fn ->
    # lots of mocks
  end)
end

# lots of tests

test "a new thing" do
  Tesla.Mock.mock(fn ->
    # a mock to override a previously defined one
    # (to test a different response for the same request,
    # like an error)
  end)
  
  # Now all of my mocks from `setup` are gone
end

I don't know what the API would look like but I have two ideas to not break existing tests:

Option 1:

def setup do
  # `mock` now returns a collection of all mocks
  Tesla.Mock.mock(fn ->
    # lots of mocks
  end)
end

# lots of tests

test "a new thing",  context_mocks do
  Tesla.Mock.mock(context_mocks, fn ->
    # a mock to override a previously defined one
    # (to test a different response for the same request,
    # like an error)
  end)
  
  # Now all of my mocks from `setup` are still there, but one of them is overridden with a newer mock
end

Option 2:

def setup do
  # no change to `mock`
  Tesla.Mock.mock(fn ->
    # lots of mocks
  end)
end

# lots of tests

test "a new thing" do
  Tesla.Mock.add_mock(fn ->
    # a mock to override a previously defined one
    # (to test a different response for the same request,
    # like an error)
  end)
  
  # Now all of my mocks from `setup` are still there, but one of them is overridden with a newer mock
end

Thoughts? 😅

[yordis]

After #718 I encourage people to migrate to the new Tesla.Test module