The junit package shouldn't need to know anything about benchmarks and
gtr.Benchmark will be removed in a future commit. Instead, it will be
the responsibility of the gotest parser to represent benchmarks using
gtr.Test.
Benchmark durations were originally formatted with higher precision
because they contained just the NsPerOp field. Now that we're
approximating the total duration they are more accurate and don't need
as many decimals.
Having multiple benchmarks for a single name is something specific to
the gotest output, so grouping benchmarks by name is not something that
belongs in the junit package.
