This is a follow-up to patch #8770.

This patch adds the rest of automated test cases on Gnash's
`ExternalInterface.addCallback(methodName,instance,method)`
listed in bug #37223 comment 7 to Gnash testsuite:

• Registration test with `method=null` and `method=undefined`: callback registration must fail to pass.
• Registration test with `instance=null` and `instance=undefined`: callback registration must succeed to pass.
• Test on `this` inside callback when `instance!=null`: must be user-specified Object to pass.
• Test on `this` inside callback when `instance=null` and `instance=undefined`: must be a value equals to `null` and `undefined` to pass.
• Test on `this` inside callback when `instance=null` and `instance=undefined`: must be an object similar to `this` in a function called via `.call(null)` (or `.call(undefined)`) to pass. (Expected to fail)
• Long callback arguments test ("The","quick","brown","fox","jumps","over","the","lazy","dog"): must be passed correctly to pass.

Note about special cases where `instance=null` and `instance=undefined`:

`this` pointer inside the callback function should be the same as one inside
a function called via `function.call(null)` or `function.call(undefined)`,
which have following properties:

• `typeof()` call returns "object" (not "undefined" or "null"); currently XFAIL as it gives "null".
• having equality with `undefined`; currently PASS.
• having equality with `null`; currently PASS.
• not having strict equality with `undefined`; currently PASS.
• not having strict equality with `null`; currently XFAIL as it is `null`.
• when appended to an empty string, will result in a string "undefined"; currently XFAIL as it gives "null" string.

Gnash: 0.8.11dev (patched against git 4c03566 19-Dec-2015)
Adobe Flash Player: 11.2 r202 (11.2.202.491) NPAPI binary
Browser: Iceweasel 10.0.12 (debian)
System: Debian GNU/Linux 7.0 Wheezy i386