phpunit: Fix omission of 'integration/' in phpunit.php runner

Follows-up 214750d8d2 which created this directory, but forgot
to add it in tests/phpunit/suite.xml.

This was only realized this week, 9 months later, as part of
I29f5e761b977c where an integration test was fixed due to
observing a failure locally when running it directly, and this
not having been caught by CI when the failure was introduced,
because the suite is not being run.

Change-Id: I9edb8fad870ec4345c102e3cc2d280f872d20a22

tests/phpunit/suite.xml

@@ -52,6 +52,9 @@
 		<testsuite name="unit">
 			<directory>unit</directory>
 		</testsuite>
+		<testsuite name="integration">
+			<directory>integration</directory>
+		</testsuite>
 	</testsuites>
 	<groups>
 		<exclude>