strconv/itoa: add test to generate the longest output string possible by formatBits

The new test case produces the longest string representation possible and thereby uses
all of the 65 bytes in the buffer array used by the formatBits function.

Change-Id: I11320c4de56ced5ff098b7e37f1be08e456573e2
Reviewed-on: https://go-review.googlesource.com/2108
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>