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

This reverts commit 6f2c1682a724e0af515a4c9256451cfa60e77637.

phpunit.xml.dist

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