go/scanner: improve variety in benchmarks for file scanning

BenchmarkScanFile was scanning scanner.go, which makes comparison
difficult for a CL modifying that file. That file is also is not
necessarily representative syntax.

Add a few additional files as subtests to provide a larger variety of
metrics.

Change-Id: Ib78303c2546debd84a0b5478ae438ba891d9e6e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/308610


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>