Add presubmit to check if there are any files that have `import "testing"` (#2480)

The check is done through
```
if [[ $(go list -f '{{ join .Deps " "}}') == *" testing "* ]]; then
   error..
fi
```
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

.presubmit/import-testing

+#!/bin/bash
+
+echo "** presubmit/$(basename $0)"
+
+# Check to make sure "testing" is not imported
+if [[ $(go list -f '{{ join .Deps " "}}') == *" testing "* ]]; then
+    echo "** presubmit/$(basename $0): please remove any 'import \"testing\"'"
+    exit 1
+fi