Update phpunit configuration format and include filter for code coverage

Author: simensen

phpunit.xml.dist

@@ -1,7 +1,13 @@
-<phpunit bootstrap="vendor/autoload.php" colors="true">
-    <testsuites>
-        <testsuite name="dflydev FIG Cookies Tests">
-            <directory>./tests</directory>
-        </testsuite>
-    </testsuites>
+<?xml version="1.0"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" bootstrap="vendor/autoload.php" colors="true" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd">
+  <coverage>
+    <include>
+      <directory>./src</directory>
+    </include>
+  </coverage>
+  <testsuites>
+    <testsuite name="dflydev FIG Cookies Tests">
+      <directory>./tests</directory>
+    </testsuite>
+  </testsuites>
 </phpunit>