Array_UF21 does not always work with incremental SMT back-end

The test consistently fails when doing coverage checking (was also the
case in the coverage check run of #7434, which was the PR introducing this
test). The output is:
`Invalid SMT response "corrupted"`
The test appears to be working fine when not doing coverage checking,
though.

Author: tautschnig

regression/cbmc/Array_UF21/test.desc

@@ -1,4 +1,4 @@
-CORE new-smt-backend
+CORE
 main.c
 --arrays-uf-always --bounds-check
 ^VERIFICATION FAILED$