Added processIsolation flag to PHPUnit so headers_sent() begins as `false` for each test

phpunit.xml.dist

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<phpunit bootstrap="tests/bootstrap.php">
+<phpunit bootstrap="tests/bootstrap.php"
+         processIsolation="true">
     <testsuites>
         <testsuite name="Monolog Test Suite">
             <directory>tests/Monolog/</directory>