testing: be clearer about when Benchmark needs Init #77499
Description
The documentation of testing.Benchmark says that
If f depends on testing flags, then [Init] must be used to register
those flags before calling Benchmark and before calling [flag.Parse].
but doesn't specify what might depend on the flags.
I ran into this when I returned inside a for b.Loop() loop, which tried to log an error, but instead panicked because logging accesses the fullPath flag.