* Express JSON syntax tests with multiple assertions

When written with a single assertion, this test verifies only that "one
or more" of the Unicode points produces a SyntaxError. Re-factor the
test so that parsing behavior for each code point can be tested in
isolation.

* Re-introduce assertion

ECMAScript edition 2016 relies on the Unicode standard at version 8.0.0.
As part of a larger effort to update Test262 accordingly, the assertion
for the SyntaxError resulting from the use of the Mongolian Vowel
Separator (U+180E) was removed. However, ECMA-401 (The JSON Data
Interchange Format) continues to describe "insignificant whitespace" as
follows:

> Insignificant  whitespace  is  allowed  before  or  after  any  token.
> The  whitespace  characters  are:  character  tabulation  (U+0009),
> line  feed  (U+000A),  carriage  return (U+000D),  and  space
> (U+0020)

And because ECMA262 specifies step 2 of the `JSON.parse` algorithm as
follows:

> 2. Parse JText interpreted as UTF-16 encoded Unicode points (6.1.4) as
>    a JSON text as specified in ECMA-404. Throw a SyntaxError exception
>    if JText is not a valid JSON text as defined in that specification.

...the use of U+180E between JSON tokens should continue to produce a
SyntaxError, regardless of ECMA262's updated Unicode standard.

* fixup! Express JSON syntax tests with multiple assertions