selftest/static-group: Add seat

With a group added to the seatd recipe, we need an entry in our
static group file for selftest.

(From OE-Core rev: 655d334584769a9b9d86cf8fab2c06bc0878d0b3)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-selftest/files/static-group

@@ -27,3 +27,4 @@ render:x:527:
 sgx:x:528:
 ptest:x:529:
 xuser:x:530:
+seat:x:531: