For issue #657 Unit tests should check various number formats

This adds tests for various number formats (namely: decimal with and without a decimal point, exponent with both capital and lowercase E, both in the positive and negative) for the various get* and opt* number methods of JSONObject.

It does NOT add hex, floating point hex, or octal, as these are not supported JSON features (see https://stackoverflow.com/a/52671839/6323312). JSONObject also does not support hex, floating point hex, or octal.

I can add these additional number format tests if wanted, though.