Reduce memory consumption of cell_stub.rs test

With MiniSat the test takes 45 seconds instead of 30 seconds, but only
consumes 5.5 GB of memory instead of 9 GB. This will hopefully fix CI
failures.

tests/expected/function-contract/interior-mutability/api/cell_stub.rs

@@ -1,6 +1,6 @@
 // Copyright Kani Contributors
 // SPDX-License-Identifier: Apache-2.0 OR MIT
-// kani-flags: -Zfunction-contracts
+// kani-flags: -Zfunction-contracts --solver minisat
 
 /// The objective of this test is to show that the contracts for double can be replaced as a stub within the contracts for quadruple.
 /// This shows that we can generate `kani::any()` for Cell.