For the open once, read many test in the benchmarking code - consider combining the open, read(s), and close tests into a single test

[briangow]

In our most recent meeting, the group generally agreed that it would probably be better to combine the open, read(s), and close tests from #94 into a single test which times open once, read many, close once. However, we decided to leave the test as it is in #94 for now because it may be informative as the engineers update their formats to work with this new test.