tools/lib/testutil: ignore "short write" error in genXUnitReportOnCmdError function.
tools/lib/testutil: ignore "short write" error in genXUnitReportOnCmdError function.

Also use a larger intial buffer for recording stdout/stderr from the command.

(This is an attempt to fix https://github.com/veyron/release-issues/issues/862)

Change-Id: I1f271de13ada50cb1cc61390c6b8003c269ce2a0

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools