Prevent module list changes outside of setup when sharing the test environment

Problem/Motivation

Both #3629052: [PP-1] Share caches between kernel test methods and #3623409: [PP-1] Add the "ShareEnvironment" attribute to all Kernel tests independently landed on the realization that changing the module list within test methods is problematic when sharing the test environment, because it may break assumptions about available/installed plugin definitions and entity/field types, thus interfering with the environment setup process.

Proposed resolution

Prevent module list changes from happening outside the environment setup phase.

Remaining tasks

• Validate the proposed solution.
• Implementation
• Reviews