testing: fix fractional ns/op printing

CL 166717 changed the way ns/op benchmark results were printed and
inadvertently rounded all ns/op results down to an integer, even if
they were small enough to print with digits after the decimal place.
For example, prior to this change, we got output like

    BenchmarkFast-8       380491575	 3.12 ns/op

CL 166717 changed this to

    BenchmarkFast-8       380491575      3.00 ns/op

This had the further side-effect that ns/op values between 0 and 1
would not be printed at all because they would be rounded down to 0.

This CL fixes this by always recomputing the float64 value of ns/op
instead of using the int64 truncation from BenchmarkResult.NsPerOp.

Fixes #30997. Fixes #31005.

Change-Id: I21f73b9d5cc5ad41e7ff535675d07ca00051ecd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/168937


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>