Update reference from db:test:prepare to use test:db instead

This reference to `db:test:prepare`, which was made private in fa15111 and covered in rails#2560 is outdated.

We should teach users about the public task instead (using `bin/rails --help` as a guide).

Co-authored-by: Emil Kampp <[email protected]>
Co-authored-by: zzak <[email protected]>

Author: ekampp

guides/source/testing.md

@@ -702,7 +702,7 @@ the migrations against the development database (`bin/rails db:migrate`) will
 bring the schema up to date.
 
 NOTE: If there were modifications to existing migrations, the test database needs to
-be rebuilt. This can be done by executing `bin/rails db:test:prepare`.
+be rebuilt. This can be done by executing `bin/rails test:db`.
 
 ### The Low-Down on Fixtures