add tests for untrusted tampering with data and metadata

This is a test-driven approach to investigating and fixing the issue
mobilecoinfoundation#1576

These tests confirm that the trusted side indeed does not always
detect when untrusted tampers with the memory, at this revision
I have these results:

```
running 5 tests
test helpers::exercise_oram_storage_clear_data ... FAILED
test helpers::exercise_oram_storage_hammer_data ... FAILED
test helpers::exercise_oram_storage_hammer_metadata ... ok
test helpers::exercise_oram_storage_clear_metadata ... FAILED
test helpers::exercise_oram_storage_shims ... ok
```

So the shims work normally when the memory is not tampered with,
and the code detects when the untrusted metadata is hammered.
But it does not detect when it is cleared, or when the data is
tampered with.

This is likely because of the issue point out in mobilecoinfoundation#1576, in the
next commits we will fix it and get all these tests to pass.

fog/ocall_oram_storage/trusted/Cargo.toml

@@ -26,3 +26,7 @@ blake2 = { version = "0.9.2", default-features = false, features = ["simd"] }
 
 [target.'cfg(not(any(target_feature = "avx2", target_feature = "avx")))'.dependencies]
 blake2 = { version = "0.9.2", default-features = false }
+
+[dev-dependencies]
+lazy_static = "1.4"
+mc-util-test-helper = { path = "../../../util/test-helper" }

fog/ocall_oram_storage/trusted/README.md

@@ -31,7 +31,7 @@ An important optimization is used:
 - If the ciphertext metadata from untrusted is all zeroes, then we assume this is
   the first time we access the block, and pass all zero metadata and data back
   to the Oblivious RAM. Effectively, we lazily zero the data segments in blocks
-  living untrusted memory memory, and dont require that we write encryptions of
+  living untrusted memory, and dont require that we write encryptions of
   zeros to every memory cell at initialization.
 
 Authentication design