Problem/Motivation
This was discovered while working on #3332593: Adopt PluginExists validator in relevant places.
It turns out that there are a good number of views in core (mostly test views, but some are part of modules' shipped config) that reference invalid handler plugin IDs. Apparently these are converted to supported plugin IDs at runtime (or install time? I'm not really sure), but that still means the config we're shipping is invalid.
At the moment, the only reason tests don't fail is because we are allowing Views config schema to accept invalid plugin IDs, and considering the fallback plugin IDs as valid, just so we don't have to fix all those views in #3332593: Adopt PluginExists validator in relevant places. That work should be done in a separate scope (this issue).
Steps to reproduce
If you remove the allowFallback: true lines from Views' config schema, you'll start to get test failures due to various views using invalid handler plugin IDs.
Proposed resolution
Remove all of the allowFallback: true lines and fix the views which cause trouble.
Remaining tasks
Do that, and commit it.
