ViewExecutableTest uses a mocked argument callback wrongly (and fails in PHPUnit8)

Found in #3063887: Support PHPUnit 8 optionally in Drupal 8, while keeping support for ^6.5 and ^7.

Problem/Motivation

ViewExecutableTest has mock expectation fails in PHPUnit8 due to callback return type should be bool and not string.
The callback return type is signed for PHPUnit 8, and it currently returns a string being the argument passed in - which is not necessary.

Proposed resolution

Use $this->anything() as expected argument in place of the callback.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet