Fix errors in benchmark test cases

[kenya-sk]

Errors exist in the benchmarks tests. Running test_benchmarks.sh results in the following three types of errors.
These issues have already been identified and discussed here #991 (comment).

================================================================================
INFO: Elapsed time: 51.836s, Critical Path: 49.69s
INFO: 441 processes: 6 internal, 435 local.
INFO: Build completed, 3 tests FAILED, 441 total actions
//benchmarks/scripts:benchmark_util_test                                 PASSED in 7.6s
//benchmarks/scripts:benchmark_clifford_circuit                          FAILED in 5.6s
//benchmarks/scripts:benchmark_op_gradients                              FAILED in 5.9s
//benchmarks/scripts:benchmark_random_circuit                            FAILED in 5.6s

[kenya-sk]

Hi, @mhucka,
I'd like to work on this issue, as we discussed. If that's okey, could you assign it to me.