References: A. Twitter conversation between myself and @WyriHaximus https://twitter.com/shaunbramley/status/765939778277806080
B. reactphp/child-process#5
C. sebastianbergmann/phpunit#2194

As per @WyriHaximus request within Ref A, I am submitting an issue that affects all reactphp test suites.

As per Ref C, unit tests that execute getMock() with PHPUnit 5.4+ will receive warning messages for each call indicating that the method has been deprecated, as evidenced at Ref B.

Absence of PHPUnit from require-dev, results in the expectation that unit tests will run error free with latest tooling. Yet, when latest tooling is used, tests throw warnings (unexpected behaviour). Part of the issue seems to stem from Ref B, which highlights a decision to avoid referencing PHPUnit as a developmental requirement.