Extract JSONSharedTestCases

[kamipo]

Both mysql2/json_test.rb and postgresql/json_test.rb have same test
cases.

[matthewd]

Perhaps we should pull this up as test/cases/json_test.rb, with the ActiveRecord::Base.connection.supports_json? condition to decide whether it runs for the current adapter? That feels like the more common pattern.

[kamipo]

A reason that extracting JSONSharedTestCases is for sharing the test cases with Mysql2JSONTest, PostgresqlJSONTest, and PostgresqlJSONBTest.
And also, the result of connection.supports_json? is true for json, but not true for jsonb.
So I'd kept adapters/{mysql2,postgresql}/json_test.rb for now.

[matthewd]

👍🏻 I'd forgotten about jsonb.