This is motivated by WT-13880 where a session->truncate wasn't wrapped in testutil_check so the invalid API call failed silently.

WiredTiger adds WT_GCC_FUNC_DECL_ATTRIBUTE((warn_unused_result)) too all of its internal functions to prevent issues like this, but we don't set it on the public API functions of session/connection/cursor.

This is reasonable as we don't want to overly constrain external users, but it would be good to have a flag that enforces this for our testing.

The scope of this ticket is a timeboxed investigation of enabling warn_unused_result - or an alternate flag that does the same thing - for our csuite and cppsuite testing.
We need to make sure this flag is only enabled when explicitly requested by a user. i.e. this flag should not be enabled by setting HAVE_DIAGNOSTIC.