itest: add missing break statements to evalexp()

The commit mentioned below replaced return statements inside a switch so
that other code could be called after the switch. However, it didn't add
any break statements, causing the cases to run together. Fix this.

Reported-by: Coverity (CID 132282, 132283)
Fixes: 7861204c ("itest: make memory access work under sandbox")
Signed-off-by: Stephen Warren <swarren@nvidia.com>