strconv: avoid truncation of output in parse int tests

strconv: avoid truncation of output in parse int tests

If needed cast the test table values to a higher bit size
integer type instead of casting the result values of the
tested function to a lower bit size integer type.

Change-Id: Iaa79742b2b1d90c7c7eac324f54032ebea0b1b41
Reviewed-on: https://go-review.googlesource.com/55137
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>