(A stripped down version of) $GOROOT is uploaded to the device
before running standar library tests, including many (all?)
testdata directories.

Use the --sync flag when pushing testdata directories to the device
in case it is already present.

Change-Id: If8104f9d15838c1be3623adcf831a7188303c376
Reviewed-on: https://go-review.googlesource.com/c/164338Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>