Enforce $modules property as protected in test-classes

Problem/Motivation

#2822382: Make every $modules property protected on classes extending BrowserTestBase and KernelTestBase → Set all $modules properties as protected.
#2983395: user module's flood controls should do better logging → Inadvertently reverted this by adding new test-classes.

Proposed resolution

Investigate adding a way of detecting that a new test has overridden this property and set the wrong visibility.
This may end up being a phpcs rule.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet