We currently use WT_DECL_RET to declare the ret variable in our test/cppsuite code, but this is an internal macro for WiredTiger. We should limit our interaction with WiredTiger to only the public api.

This ticket will replace all uses of WT_DECL_RET with a normal `int ret = 0;` call. during this ticket we should also check we haven't used other internal macros such as WT_ERR or WT_RET.

Definition of done:
`grep -r "WT_BILLION|WT_DECL_RET|WT_IGNORE_RET" wiredtiger/test/cppsuite` returns zero results
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