make `db:structure:load` listed with `rake -T`
docs, restructure newly added part to `includes`. [ci skip]

This is a follow up to #14139.

/cc @carlosantoniodasilva
Merge pull request #14952 from gregmolnar/schema_load

make `db:structure:load` listed with `rake -T`
Refactor test to not care about the specific result of valid?

Only care about its truthiness rather than asserting specific true/false
values. If we need to check for the return value in particular, there will
be a test for that.
Remove duplicated fixture addition
Reorganize ActiveRecord tasks [Arun Agrawal & Abd ar-Rahman Hamidi]
Merge pull request #14940 from arunagw/hbakhtiyor-test-db-namespaces-ar

Reorganize ActiveRecord's rake tasks
Revert "Merge pull request #14940 from arunagw/hbakhtiyor-test-db-namespaces-ar"

This reverts commit fe9859600824268ff614cb3eb48b62c7165a1fbd, reversing
changes made to add9560643ef79830febf47fa1228752d1df3502.