refactor: remove event faking apis from public api

[crisbeto]

Moves the event faking APIs into cdk/private/testing in order to avoid exposing them in the public API of cdk/testing.

Resolves COMP-239.

[crisbeto]

@mmalerba @jelbourn @devversion I've reworked it so all the private APIs are in cdk/testing/private and cdk/testing/private/e2e now.

[mmalerba]

LGTM, seems reasonable to me, but @devversion should confirm that with this structure:

1. @angular/cdk/testing/private and @angular/cdk/testing/private/e2e will not be published to NPM
2. The imports of the private symbols in @angular/cdk/testing will still work

[devversion]

This won't work in the release output since cdk/testing/private will not be part of the release output. We could fix that by making it a tertiary entry-point (with bundles etc.), but then cdk/testing/ also needs to import it through the module name (i.e. @angular/cdk/testing/private).

Also, IIRC we chatted about this again in the Slack channel. The solution Jeremy proposed initially seems to work the best without having to create a new published entry-point that contains all private testing utilities.

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