internal/strconv: add tests and benchmarks for ftoaFixed
ftoaFixed is in the next CL; this proves the tests are correct
against the current implementation, and it adds a benchmark
for comparison with the new implementation.
Change-Id: I7ac8a1f699b693ea6d11a7122b22fc70cc135af6
Reviewed-on: https://go-review.googlesource.com/c/go/+/717181
Auto-Submit: Russ Cox <rsc@golang.org> Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>