Potentially import more JSON tests

[gsnedders]

@nst has written a a nice JSON testsuite; this targets RFC 7159's definition of JSON rather than ECMA-404's, and obviously many tests don't apply given the source for JSON.parse is a string primitive rather than a bunch of bytes. We should, nevertheless, check that we have coverage for all of their tests that apply.

[devsnek]

So far I found at least these cases aren't tested in test262:

• -2.
• -01
• 0.e1
• 2.e-3
• 2.e+3
• 2.e3
• -012
• 012
• 1.

At this point would it make sense to still work on integrating JSONTestSuite into test262, or just add tests we find manually? (In the latter case, I'm not entirely sure where to put them)