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

[mmalerba]

• Add label filter for MatTabHarness
• Add ability to filter returned harnesses in MatTabGroupHarness#getTabs
• Add a method for selecting a child tab to the MatTabGroupHarness
• Remove getContentElement and replace it with getHarnessLoaderForContent

@jelbourn @devversion Curious about your opinions: My reason for removing getContentElement is that I want to avoid returning TestElement objects from our harnesses. I don't think they're particularly useful to test authors, and instead we probably want APIs that are more focused in purpose. I do think it makes sense, however, to allow users to get some handle on the content. I think the thing users are most likely to do with the content element is either query for more harnesses within it, or query outside of the harness system (e.g. document.querySelector). I settled on returning a selector because it allows the user to do either of those things. Another option would be to provide a method for loading a sub-harness directly (they pass the harness class and we return an instance). We either could provide this method instead of or in addition to the getSelectorForContent method.

Update: Changed getSelectorForContent to getHarnessLoaderForContent

[jelbourn]

What about getting a locator scoped to the content?

[devversion]

LGTM in general.

@mmalerba I'm somewhat hesitant in thinking that returning TestElement's is an anti-pattern. For simple assertions where someone wants to get a handle on the content element, it feels totally expected to me that a TestElement instance is returned that works for both e2e and test bed tests.

The abstraction of the elements we are dealing with in the harnesses is not only useful to implementer's of harnesses, but also to consumers of harnesses as they do not need to deal with platform-specific things (i.e. calling querySelector). I don't see any big impact on providing such a harness method (it seems like it can be quite useful and commonly used)

[mmalerba]

As discussed on slack, switch to returning a HarnessLoader for the tab content, PTAL.

[jelbourn]

LGTM

[devversion]

Nice! I really like that harness loader method 🎉

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