Less aggressive feature tests

The parse feature tests currently detect and ban extensions to the JSON grammar:

// Safari <= 5.1.2 and FF 3.1b1 allow unescaped tabs in strings.

// FF 4.0 and 4.0.1 allow leading `+` signs and leading
// decimal points. FF 4.0, 4.0.1, and IE 9-10 also allow
// certain octal literals.

// FF 4.0, 4.0.1, and Rhino 1.7R3-R4 allow trailing decimal
// points. These environments, along with FF 3.1b1 and 2,
// also allow trailing commas in JSON objects and arrays.

Let's consider removing these. They add weight, test for edge cases, and penalize modern environments. JSON that includes trailing decimals, commas, or octal literals is already non-portable.

[bnjmnt4n]

Sure.