Move phpunit.xml into root to make PHPUnit easier to run

This seems to be pretty standard. Can just run `vendor/bin/phpunit` without any arguments. Removes the need for `composer test` (which is not ideal anyway as it removes colours from the output).

framework/core/phpunit.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<phpunit backupGlobals="false"
+         backupStaticAttributes="false"
+         colors="true"
+         convertErrorsToExceptions="true"
+         convertNoticesToExceptions="true"
+         convertWarningsToExceptions="true"
+         processIsolation="false"
+         stopOnFailure="false"
+         syntaxCheck="false">
+
+    <testsuites>
+        <testsuite name="all">
+            <directory suffix="Test.php">./tests</directory>
+        </testsuite>
+    </testsuites>
+
+</phpunit>