Fix xmllint error on phpunit.legacy.xml

Copies the legacy config rather than moving it so that xmllint can scan
it, which was the last thing preventing the 7.x build from passing.

Since the config is always available under the same filename, we also no
longer need to pass a custom `--configuration` flag for phpunit.

.travis.yml

@@ -36,16 +36,12 @@ install:
   # different config infection uses for individual mutations
   - if [[ ${TRAVIS_PHP_VERSION:0:1} == "7" ]]
     && [ ${TRAVIS_PHP_VERSION:2:1} -lt 4 ]; then
-      mv -v tests/phpunit.legacy.xml tests/phpunit.xml
+      cp -v tests/phpunit.legacy.xml tests/phpunit.xml
     ; fi
 
 script:
   - vendor/bin/grumphp run
-
-  - if [[ -z "$PHPUNIT_LEGACY" ]]; then composer test; else
-      vendor/bin/phpunit --testdox --color=always
-        --configuration tests/phpunit.legacy.xml
-    ; fi
+  - composer test
 
   - if [[ -z "$PHPUNIT_LEGACY" ]]; then composer infection; else
       vendor/bin/infection --ansi --threads=4 --coverage=tests/test-results