tf_fuzz: fix sst `check ... except fail` calls

Fix the error code modelling of sst calls with data value checks (`check
...`), and except fail.

Currently, demo/8.test erroneously fails:

----
set sst name snortwaggle data *;
read sst name snortwaggle check "almost certainly not *this*" expect fail;
----

When reading `expect fail`, the sst read call is marked as "should
fail". However, the intended behaviour is for the read to succeed and
the check to fail.

To fix this, this patch adds a template variable, $check_expect, that
can be used to modify the expected value of the check (similar to how
$expect is used for expected values of calls). Using this, separate
pass/fail values can be set for the check as well as the call itself.

Change-Id: Ia55b6f2fef5737d67540e98ac6fc5e7695a8515f
Signed-off-by: Nik Dewally <Nik.Dewally@arm.com>
diff --git a/tf_fuzz/tfz-cpp/utility/data_blocks.hpp b/tf_fuzz/tfz-cpp/utility/data_blocks.hpp
index bee4594..723ee95 100644
--- a/tf_fuzz/tfz-cpp/utility/data_blocks.hpp
+++ b/tf_fuzz/tfz-cpp/utility/data_blocks.hpp
@@ -43,7 +43,7 @@
         bool pf_nothing;  // true to not generate results-check(s)
         bool pf_pass;  // if !expect.pf_nothing, then pass is expected
         bool pf_fail;  // if "expect fail" was specified
-        bool pf_specified;
+        bool pf_specified; // if "expect <ERROR_CODE>" was specified
             /* if !pf_nothing && !pf_pass, then
                true == expected result was specified
                false == tf_fuzz must model expected result, and
@@ -185,7 +185,7 @@
         bool can_encrypt;  // OK for encryption (fail other uses).
         bool can_decrypt;  // OK for decryption (fail other uses).
         bool can_sign;     // OK for signing (fail other operations).
-        bool can_verify;   // OK for verifing a message signature (fail other uses).
+        bool can_verify;   // OK for verifying a message signature (fail other uses).
         bool derivable;    // OK for derive other keys (fail other uses).
         bool persistent;   // must be deleted at the end of test.
         string usage_string;