Fix unused variables in search near cpp tests

Three issues have been detected in the search_near_01.cxx test:

Parse warning (ID: 120809)

A parse warning may reduce the maintainability of the code, or it may indicate a serious error. A parse warning from the Coverity parser may indicate a bug or poor coding practice
/test/cppsuite/tests/search_near_01.cxx:58: PW.DECLARED_BUT_NOT_REFERENCED 120809 variable "cmpp" was declared but never referenced

Unchecked return value (ID: 120810 and 120811)
If the function returns an error value, the error value may be mistaken for a normal value.

In search_near_01::​read_operation(test_harness::​thread_context *): Value returned from a function is not checked for errors before being used.

Calling commit without checking return value (as is done elsewhere 4 out of 8 times).

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One issue has been detected in the search_near_02.cxx test:

Parse warning (ID: 120816)

declared_but_not_referenced: variable "ret" was declared but never referenced /test/cppsuite/tests/search_near_02.cxx:270.