fix(mock-doc): make MockAttributeMap iterable

[simonhaenisch]

TL;DR Proposed changes:

• implement [Symbol.iterator] in MockAttributeMap to make it iterable
• implement get [Symbol.toStringTag] in MockAttributeMap
• don't pass symbols into isNaN in attribute proxy
• prettier formatted a couple things

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When writing E2E tests I ran into different PrettyFormatPluginError errors, when Jest would try to pretty-format an E2EElement while iterating over its attributes:

PrettyFormatPluginError: Cannot read property 'name' of undefinedTypeError: Cannot read property 'name' of undefined

or

PrettyFormatPluginError: _val$hasAttribute.call is not a functionTypeError: _val$hasAttribute.call is not a function

at node_modules/jest-matcher-utils/node_modules/pretty-format/build/plugins/DOMElement.js

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This error might only happen in my case because I'm using jest@26 and I think Stencil still defaults to jest@25, but doesn't complain if a higher major version is used cause it's likely compatible.

Anyway, after debugging this for half a day, I figured out that pretty-format fails to print the attributes because MockAttributeMap is not really iterable but it tries to iterate it (passed into Array.from, see facebook/jest/pretty-format/src/plugins/DOMElement.ts#L87 and facebook/jest/pretty-format/src/plugins/DOMElement.ts#L87), and ends up being an Array of undefineds (not sure why).

The solution is to add an implementation of [Symbol.iterator]. Another symbol that pretty-format tries to access is Symbol.toStringTag so I've also implemented that.

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Also, the attribute proxy uses isNaN which breaks when called on a symbol:

PrettyFormatPluginError: Cannot convert a Symbol value to a numberTypeError: Cannot convert a Symbol value to a number
        at isNaN (<anonymous>)

      at Object.get (node_modules/@stencil/core/mock-doc/index.cjs:21:10)
          at Proxy.toString (<anonymous>)
          at Array.map (<anonymous>)

thus I've added a check for that.

[simonhaenisch]

I've confirmed that this test works by removing my [Symbol.iterator] implementation. Array.from(map) would then yield [undefined] instead of [MockAttr].