Copy unittests binary in android-sync.sh

Without this change, `make android_arm.release.check` fails since the
unittests binary does not exist on the device.

BUG=v8:3695
LOG=

Review URL: https://codereview.chromium.org/722373003

Cr-Commit-Position: refs/heads/master@{#25405}

tools/android-sync.sh

@@ -88,6 +88,7 @@ function sync_dir {
 echo -n "sync to $ANDROID_V8/$OUTDIR/$ARCH_MODE"
 sync_file "$OUTDIR/$ARCH_MODE/cctest"
 sync_file "$OUTDIR/$ARCH_MODE/d8"
+sync_file "$OUTDIR/$ARCH_MODE/unittests"
 echo ""
 echo -n "sync to $ANDROID_V8/tools"
 sync_file tools/consarray.js