cli exit tests: valid wasi commands must export a memory

wiggle enforces this but the specially-overridden proc_exit
function did not. Now that we proc_exit through wiggle, wiggle
will trap if it cannot import the instance's memory

tests/wasm/exit_with_saved_fprs.wat

@@ -3,6 +3,7 @@
     (type (func (param i32)))
     
     (import "wasi_snapshot_preview1" "proc_exit" (func (type 0)))
+    (memory (export "memory") 0)
     
     (func $exit (param i32)
         local.get 0