Intl: Add tests for subclassing Locale, ListFormat and RelativeTimeFormat.

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[javascriptcore-test262-automation] Updated curation log with latest revision sha's from export and changed files.
    sourceRevisionAtLastExport: a2f7b0c562 targetRevisionAtLastExport: 5594c916

[javascriptcore-test262-automation] Changes from https://github.com/webkit/webkit.git at sha a2f7b0c562 on Fri Sep 07 2018 18:19:35 GMT+0000 (Coordinated Universal Time)

Fixes #1705.

Import test changes from JavaScriptCore

[javascriptcore-test262-automation] Updated curation log with latest revision sha's from export and changed files.
    sourceRevisionAtLastExport: 3ced8564a6 targetRevisionAtLastExport: f8c62a49

[javascriptcore-test262-automation] Changes from https://github.com/webkit/webkit.git at sha 3ced8564a6 on Thu Sep 06 2018 18:18:56 GMT+0000 (Coordinated Universal Time)

Import test changes from JavaScriptCore

[javascriptcore-test262-automation] Updated curation log with latest revision sha's from export and changed files.
    sourceRevisionAtLastExport: ce279990f8 targetRevisionAtLastExport: 170f4cbb

[javascriptcore-test262-automation] Changes from https://github.com/webkit/webkit.git at sha ce279990f8 on Wed Sep 05 2018 18:18:55 GMT+0000 (Coordinated Universal Time)

Import test changes from JavaScriptCore

[javascriptcore-test262-automation] Updated curation log with latest revision sha's from export and changed files.
    sourceRevisionAtLastExport: 3d4f832de4 targetRevisionAtLastExport: 199f6b31

[javascriptcore-test262-automation] Changes from https://github.com/webkit/webkit.git at sha 3d4f832de4 on Tue Sep 04 2018 18:18:22 GMT+0000 (Coordinated Universal Time)

Fix feature tags and trailing space and Generate tests

Import test changes from JavaScriptCore

test: Add private generator method tests (#1343)

Add missing features tags for class fields templates and cases

Update the recommended invocation of `pip` to use the `python` command,
avoiding discrepancies on systems where the `pip` and `python` commands
are not provided by the same Python installation. In the interest of
consistency, also update the configuration for the project's continuous
integration service.

Recommend additional tooling for users with more advanced needs.