PHPUnit code coverage generation causing memory exhaustion
By reducing the number of files included within code coverage, as well as increasing the overall memory limit in PHP, I was able to basically kill this error. The entire Zend Framework was being included within code coverage, which is very large.
Do you have xdebug profiling enabled, if so try disabling it. I've experienced this problem before, and it came down to extensions in php (specifically xdebug profiling and/or Inclued heirarchy viewer)
As of 2019, you can use the PCOV driver with PHPUnit to generate your code coverage report. In my experience, it's only marginally less performant than running a plain PHPUnit suite.
Read Speed up PHPUnit Code Coverage Analysis for some good benchmark comparing XDebug, PHPDebug and PCOV. It also has instructions on how to enable PCOV on PHPUnit 8.
Read Setup PHP PCOV for 5 times faster PHPUnit code coverage for instructions on setting up PCOV on PHPUnit7 and below.