Extract AR::Core#inspect and AR::Core.inspect's test code from base_test.rb

The methods got moved to core.rb in commit
b2c9ce341a1c907041f55461aefebb0321280cb5, but the tests never did.
Extract AR::AttributeMethods#attribute_for_inspect's test code out from base_test.rb

The method itself got extracted out from ActiveRecored::Base in commit
ceb33f84933639d3b61aac62e5e71fd087ab65ed, but the test code never did.
Extract AR::Persistence#becomes's test code out from base_test.rb

The method got extracted out from AR::Base in commit
d916c62cfc7c59ab6411407a05b946d3dd7535e9, but the tests never did.
Merge pull request #10714 from tkhr/extract_test_from_base_test.rb_in_AR

Extract tests code out from AR's base_test.rb