Refactor AR's validations_test.rb

The `:context` switch feature is implemented in
ActiveRecord::Validations#valid? method, so we should rename the test
names, and execute `valid?` in the test.

Change test name in AR's validations_test.rb

This test is testing save method's code
docs, make `ActionNotFound` public API. [ci skip]

This is a follow up to #15058.

This exception is regularly raised during development. This means it will enter
the user realm. We should provide an API page to show that this exception is public API.

/cc @schneems
Make TimeZone#parse behave more like Time#parse.

Namely, if the mday is omitted but any other upper components are, then instead
of supplying the mday from the current time, it defaults to 1.
Tidy up implementation of #15010
Replace :base with :in [ci skip]
Merge pull request #15059 from marcioj/patch-1

Replace :base with :in [ci skip]
Merge pull request #10662 from take/change-test-name-for-ActiveRecord--Validations#valid-

Refactor AR's validations_test.rb