nearly there then, let the cleanup begin

.travis.yml

@@ -21,17 +21,32 @@ cache:
 
 install:
   - travis_retry composer install $COMPOSER_OPTS
+
+  # PHPUnit 9 supports 7.3, but Infection PHP 18 can't read
+  # its config so instead we downgrade to PHPUnit 8.5 here.
   - if [[ ${TRAVIS_PHP_VERSION:0:3} == "7.3" ]]; then
       travis_retry composer require -W phpunit/phpunit:^8.5
     ; fi
-  - if [[ ${TRAVIS_PHP_VERSION:0:1} == "7" ]] && [ ${TRAVIS_PHP_VERSION:2:1} -lt 4 ]; then
+
+  # May not need this? Now that it seems to be working, see if
+  # the --test-framework-options="-c tests/phpunit.legacy.xml"
+  # method will work or if its still broken because of the
+  # different config infection uses for individual mutations
+  - if [[ ${TRAVIS_PHP_VERSION:0:1} == "7" ]]
+    && [ ${TRAVIS_PHP_VERSION:2:1} -lt 4 ]; then
       cp -v tests/phpunit.legacy.xml tests/phpunit.xml
     ; fi
 
 script:
   - vendor/bin/grumphp run
   - composer test
-  - composer infection
+
+  - if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then
+      vendor/bin/infection --ansi --threads=4
+        --initial-tests-php-options="-d xdebug.mode=coverage"
+        --only-covered --min-msi=100 --min-covered-msi=100;
+    else composer infection; fi
+
   - composer psalm
 
 after_success: bash <(curl -s https://codecov.io/bash)