misc/android: adb push --sync testdata

(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>

misc/android/go_android_exec.go

@@ -104,7 +104,7 @@ func main() {
 	run("push", os.Args[1], deviceBin)
 
 	if _, err := os.Stat("testdata"); err == nil {
-		run("push", "testdata", deviceCwd)
+		run("push", "--sync", "testdata", deviceCwd)
 	}
 
 	// Forward SIGQUIT from the go command to show backtraces from