feat(material/autocomplete/testing): polish harness API

[mmalerba]

Adds:

• Ability to search for MatAutocompleteHarness based on value
• Ability to filter getOptions with OptionHarnessFilters
• Ability to filter getOptionGroups with OptionGroupHarnessFilters
• Ability to select an option directly, without having to get the option harness

Removes:

• MatAutocompleteHarness#getAttribute (prefer more focused APIs)
• MatAutocompleteHarness#getPanel (want to avoid returning TestElement, prefer more focused APIs)
• MatAutocompleteHarness#isPanelVisible (combined into isOpen)

Renames:

• MatAutocompleteHarness#getText --> MatAutocompleteHarness#getValue
• MatOptionGroupHarness#getLabelText --> MatOptionGroupHarness#getText

[jelbourn]

LGTM aside from small nits, add merge-ready when ready

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