.gitignore: exclude PHPUnit tests related files

The following files should not be checked in:
- phpunit.phar: per documentation [1], it is PHP version-specific
- PHPUnit's cache file
- data files generated during tests execution

[1]: https://www.dokuwiki.org/devel:unittesting

.gitignore

@@ -92,3 +92,7 @@ vendor/splitbrain/php-cli/screenshot*
 vendor/splitbrain/php-cli/generate-api.sh
 vendor/splitbrain/php-cli/apigen.neon
 
+# PHPUnit tests
+phpunit.phar
+.phpunit.result.cache
+_test/data/