Add accidentally omitted test to meson build file

01e2b7f0fd02a44e introduced a test that vacuum correctly removes tuples
older than OldestXmin. The same commit was backpatched on 14-16, but 16
is the only version with meson and the test was mistakenly left off of
the recovery test meson build file. Add it now.

Discussion: https://postgr.es/m/CAAKRu_bWmMjmqL%2BOZ2duEQ80u7cRvpsExLNZNjzk-pXX5skwMQ%40mail.gmail.com

src/test/recovery/meson.build

@@ -44,6 +44,7 @@ tests += {
       't/036_truncated_dropped.pl',
       't/037_invalid_database.pl',
       't/039_end_of_wal.pl',
+      't/043_vacuum_horizon_floor.pl',
     ],
   },
 }