This commit restores the test deleted in
cd22ecbfc2
Active Job should not test things about autoloading, this would
belong to the railties test suite probably. However, there, it feels
a bit too distant from here.
Imperfect, but on a second thought I believe this trade-off is better.
AJ infers adapater class names, and loads them. How are those classes made
available to AJ is a user's concern.
Perhaps they loaded the adapter with require, perhaps they have the class in the
autoload_once_paths. It does not matter, it is the user responsibility to make
the class available _somehow_, and AJ can assume that.
The `ActiveJob::TestHelper` replace the adapter to test adapter in
`before_setup`. It gets the target class using the `descendants`, but if
the test target job class is not loaded, will not be a replacement of
the adapter.
Therefore, instead of replacing with `before_setup`, modified to
replace when setting adapter.
Fixes#26360
