Move the WPTs out of the credential-manager directory

[samuelgoto]

Posting this on behalf of @marcoscaceres:

It seems that it would make Webkit's integration with WPT much easier if we moved FedCM's web platform tests out of the credential management [1] directory and into something else.

That seems to make general sense, given that both WebAuthn [2] and Digital Credentials [3] are tested outside too.

[1] https://github.com/web-platform-tests/wpt/tree/master/credential-management
[2] https://github.com/web-platform-tests/wpt/tree/master/webauthn
[3] https://github.com/web-platform-tests/wpt/tree/master/digital-credentials