Testing: Use SCRIPT_DEBUG in end-to-end test environment

[aduth]

Previously: #14371 (comment)
Blocked by: #11360

The end-to-end test suite includes automatic test failures for console error logging. By using development-mode scripts, we can receive more messages (and more helpful messaging) from some vendor dependencies such as React which provide warning messages in development environments.

This is currently blocked by #11360, since there are currently known warnings which occur in development environments related to the use of unsafe lifecycle methods. These must first be resolved before SCRIPT_DEBUG can be activated by default.

[aduth]

As #11360 is now closed, it's technically not blocked any longer. I suspect there may still be some rogue errors which occur in some components (see also #14851, #15790).