New plugin: Background Update Tester

New plugin: Background Update Tester, from @dd32 and me. It does what it says… Most sites are able to apply updates in the background. This plugin checks your site for compatibility and explains any problems.

After activating this plugin, visit the Dashboard → Update Tester screen. (If you are using multisite, visit Updates → Update Tester in the network admin.)

Feedback and enhancement requests welcome. (I think we should add the ability to send a test email to see that your install can email you.) This is an early rough cut.

#3-7, #dev-notes