Allow PHPUnit 8 in tests

.gitignore

@@ -1,3 +1,4 @@
+/.phpunit.result.cache
 /clover.xml
 /coveralls-upload.json
 /phpunit.xml

.travis.yml

@@ -17,6 +17,7 @@ matrix:
     - php: 7.1
       env:
         - DEPS=locked
+        - LEGACY_DEPS="phpunit/phpunit"
         - CS_CHECK=true
         - TEST_COVERAGE=true
     - php: 7.1
@@ -45,9 +46,10 @@ before_install:
   - if [[ $TEST_COVERAGE != 'true' ]]; then phpenv config-rm xdebug.ini || return 0 ; fi
 
 install:
-  - travis_retry composer install $COMPOSER_ARGS
+  - travis_retry composer install $COMPOSER_ARGS --ignore-platform-reqs
+  - if [[ $LEGACY_DEPS != '' ]]; then travis_retry composer update $COMPOSER_ARGS --with-dependencies $LEGACY_DEPS ; fi
   - if [[ $DEPS == 'latest' ]]; then travis_retry composer update $COMPOSER_ARGS ; fi
-  - if [[ $DEPS == 'lowest' ]]; then travis_retry composer update --prefer-lowest --prefer-stable $COMPOSER_ARGS ; fi
+  - if [[ $DEPS == 'lowest' ]]; then travis_retry composer update $COMPOSER_ARGS --prefer-lowest --prefer-stable ; fi
   - if [[ $TEST_COVERAGE == 'true' ]]; then travis_retry composer require --dev $COMPOSER_ARGS $COVERAGE_DEPS ; fi
   - stty cols 120 && composer show
 

composer.json

@@ -28,7 +28,7 @@
     },
     "require-dev": {
         "malukenho/docheader": "^0.1.6",
-        "phpunit/phpunit": "^7.0.2",
+        "phpunit/phpunit": "^7.5.16 || ^8.4.1",
         "zendframework/zend-coding-standard": "~1.0.0"
     },
     "suggest": {