upgrade phpunit to 8.5.8

tools/dockerfile/distribtest/php7_stretch_x64/Dockerfile

@@ -16,6 +16,6 @@ FROM debian:stretch
 
 RUN apt-get update && apt-get install -y php php-dev php-pear wget zlib1g-dev
 
-RUN wget https://phar.phpunit.de/phpunit-5.7.27.phar && \
-  mv phpunit-5.7.27.phar /usr/local/bin/phpunit && \
+RUN wget https://phar.phpunit.de/phpunit-8.5.8.phar && \
+  mv phpunit-8.5.8.phar /usr/local/bin/phpunit && \
   chmod +x /usr/local/bin/phpunit

tools/gce/linux_kokoro_performance_worker_init.sh

@@ -162,8 +162,8 @@ gem install bundler
 
 # PHP dependencies
 sudo apt-get install -y php7.2 php7.2-dev php-pear unzip zlib1g-dev
-sudo wget https://phar.phpunit.de/phpunit-5.7.27.phar && \
-    sudo mv phpunit-5.7.27.phar /usr/local/bin/phpunit && \
+sudo wget https://phar.phpunit.de/phpunit-8.5.8.phar && \
+    sudo mv phpunit-8.5.8.phar /usr/local/bin/phpunit && \
     sudo chmod +x /usr/local/bin/phpunit
 curl -sS https://getcomposer.org/installer | php
 sudo mv composer.phar /usr/local/bin/composer