slice17 was fixed in 0d7ebd5

Mark it as CORE instead of KNOWNBUG. Remove slice21 as it is exactly the
same test as slice17. This test is the example shown in issue diffblue#380,
which we can thus safely close.

Fixes: diffblue#380

Author: tautschnig

regression/goto-instrument/slice17/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.c
 --full-slice
 ^EXIT=0$