Description:
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I've been attempting to use phpdbg for code coverage reports with phpunit. I've had rather strange behaviour with it running Drupal 8's test suite. phpunit 4.8 tests simply stop running, with no error or segfault. I switched over to phpunit 5.4, so the warnings below can be ignored for now.

Under PHP 7, tests pass, but on further inspection, the assertions are not all being run correctly. For example, under php7:

Time: 14.94 seconds, Memory: 120.00Mb

OK (15 tests, 176 assertions

Under phpdbg:

Time: 5.31 seconds, Memory: 140.00Mb

OK (15 tests, 8 assertions)

This also affects PHP7.0, as I see similar behaviour there in the Ubuntu PPA for it: https://github.com/oerdnj/deb.sury.org/issues/485

Test script:
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Here's a Dockerfile to replicate this bug against the latest RC:

FROM php:7.1-rc
RUN curl -OJ https://ftp.drupal.org/files/projects/drupal-8.2.2.tar.gz
RUN tar xzvf drupal-8.2.2.tar.gz
ENV SIMPLETEST_DB="sqlite://localhost//tmp/drupal.sqlite"
CMD cd drupal-8.2.2 \
  && php vendor/bin/phpunit --verbose --debug -c core/phpunit.xml.dist --group aggregator \
  && phpdbg -rrq vendor/bin/phpunit --verbose --debug -c core/phpunit.xml.dist --group aggregator

Build it with docker build, and then use docker run <image id> and note that the assertion counts are different.