test(ngOptions): fix flaky test on Firefox 54+ and Safari 9

[gkalpak]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Reduce (remove?) test flaky-ness.

What is the current behavior? (You can also link to an open issue here)
A particular ngOptions test is very flaky on Firefox 54+ (possibly due to a (buggy?) optimization).

What is the new behavior (if this is a feature change)?
The test is more reliable on all browsers.

Does this PR introduce a breaking change?
No.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

[gkalpak]

In case anyone is wondering, this is what caused the issue (afaict):

• On Firefox and Chrome (didn't check other browsers), the selected property is implemented as a getter/setter property on the HTMLOptionElement prototype (not the <option> element itself).
• We were defining a getter/setter selected property on the <option> elements and spying on it.
• It seems that in certain cases (possibly when a specific optimization was triggered), Firefox would ignore our getter/setter spy and use the getter/setter on the prototype (potentially only for get operations).

[Narretz]

As discussed, I think we should merge this and possibly skip the test altogether on Safari where it also sometimes sporadically fails.

[Narretz]

Let's merge it as is, and see if there are Safari failures later

[gkalpak]

I disabled the test on Safari 9. Once Travis is green, I'll merge it.
(EDIT: I saw your comment too late 😁 )

[Narretz]

There still time to revert!

[gkalpak]

Too late, I squashed the commits 😛