REST API: Make plugin installation tests more robust on alternate test environments.

The REST API plugin installation tests use the `upgrader_pre_download` filter to avoid downloading the test plugin from WordPress.org. Previously, this would apply to any upgrader, which caused issues if the testing environment required a language update.

Now, the filter only overwrites the file if the `Plugin_Upgrader` is being used which should hopefully prevent the issue.

Props pfefferle, TimothyBlynJacobs.
Fixes #50671.

Built from https://develop.svn.wordpress.org/trunk@48524


git-svn-id: https://core.svn.wordpress.org/trunk@48286 1a063a9b-81f0-0310-95a4-ce76da25c4cd