Allow multiple subscribers to a MockSubscriptionLink

[dfrankland]

The behavior of MockSubscriptionLink differs from that of WebSocketLink. Specifically, MockSubscriptionLink doesn't allow multi subscriptions, which can make testing super confusing if you have a couple of components using useSubscription.

This PR adds an array of observables to MockSubscriptionLink for it to publish to when a simulateResult or simulateComplete method is called. In its current form it can have a memory leak because the array of observables is never cleaned up. This was the case prior as well, but moving forward should we remove observables that are complete or encounter an error?

[apollo-cla]

@dfrankland: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[codecov]

Codecov Report

Merging #6081 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6081   +/-   ##
=======================================
  Coverage   95.36%   95.36%           
=======================================
  Files          89       89           
  Lines        3664     3667    +3     
  Branches      903      871   -32     
=======================================
+ Hits         3494     3497    +3     
  Misses        146      146           
  Partials       24       24           

Impacted Files	Coverage Δ
.../utilities/testing/mocking/mockSubscriptionLink.ts	100.00% <100.00%> (ø)

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[hwillson]

Thanks @dfrankland!