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ARROW-11746: [Developer][Archery] Fix prefer real time check

See: https://issues.apache.org/jira/browse/ARROW-11746

Google Benchmark adds `/real_time` to the end of the benchmark name to indicate if the `real_time` observation should be preferred over the `cpu_time` observation.

https://github.com/google/benchmark/blob/af72911f2fe6b8114564614d2db17a449f8c4af0/src/benchmark_register.cc#L222

Example: `AllocateDeallocate<Jemalloc>/size:1048576/real_time`

Archery is looking for `"/realtime"`, not `"/real_time"` though.

```
 @property
 def is_realtime(self):
     """ Indicate if the preferred value is realtime instead of cputime. """
     return self.name.find("/realtime") != -1
```

Closes #9557 from dianaclarke/ARROW-11746

Authored-by: Diana Clarke <diana.joan.clarke@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>