jpm test should use temporary add-on install mechanism for beta/release

[Standard8]

Currently we're no longer able to use jpm test for add-ons on beta/release. The unbranded builds aren't easy to great via script (mozilla/mozdownload#407), so that isn't really a viable option at the moment.

It would therefore be really useful if jpm test could use the temporary add-on install mechanism for testing extensions.

[freaktechnik]

Duplicate of #468

[kumar303]

#468 is about jpm run so I can leave this one open as a reminder to also apply it to the test command. I think they will share a lot of code.