meta-selftest/files: add xuser to static-passwd/-group

As xuser-account creates a new user, we need to add it to the testing
static passwd file.

(From OE-Core rev: 6c0637a2ba7621a6398dc62b4d019cd525ee8908)

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-selftest/files/static-group

@@ -26,3 +26,4 @@ wayland:x:526:
 render:x:527:
 sgx:x:528:
 ptest:x:529:
+xuser:x:530:

meta-selftest/files/static-passwd

@@ -18,3 +18,4 @@ builder:x:522:522::/:/bin/nologin
 _apt:x:523:523::/:/bin/nologin
 weston:x:525:525::/:/bin/nologin
 ptest:x:529:529::/:/bin/nologin
+xuser:x:530:530::/:/bin/nologin