[SPARK-16515][SQL][FOLLOW-UP] Fix test `script` on OS X/Windows...

The current `sed` in `test_script.sh` is missing a `$`, leading to the failure of `script` test on OS X:
```
== Results ==
!== Correct Answer - 2 ==   == Spark Answer - 2 ==
![x1_y1]                    [x1]
![x2_y2]                    [x2]
```

In addition, this `script` test would also fail on systems like Windows where we couldn't be able to invoke `bash` or `echo | sed`.

This patch
- fixes `sed` in `test_script.sh`
- adds command guards so that the `script` test would pass on systems like Windows

- Jenkins
- Manually verified tests pass on OS X

Author: Liwei Lin <lwlin7@gmail.com>

Closes #14280 from lw-lin/osx-sed.

(cherry picked from commit d6795c7a)
Signed-off-by: Sean Owen <sowen@cloudera.com>