Increase coverage for the Intl.NumberFormat class contructor by adding
tests for checking that it calls ToObject on the locales argument.

Rework the available tests for Intl.NumberFormat.prototype.formatToParts
to improve consistency and readability.

Add an assertion to the test for default parameters in Intl.NumberFormat's
method formatToParts checking that both the values are equal to the correct
result.

Fix #1794

The spec changed in <https://github.com/tc39/proposal-intl-locale/pull/59>.

This will allow implementations to test the rest of the number formatting tests,
even if the extra precision support isn't implemented.

Add missing explicit ToString conversion for symbol values and ensure the expected NaN result is a (locale-dependent) string

The tests for the parsing of postfix increment, postfix decrement,
prefix increment, and prefix decrement were expressed using `eval`.
This made the tests more complex than necessary and also prevented the
tests from providing value to ECMAScript parsers.

Remove the use of `eval` and instead express the expectations with
literal source text.

Increase test coverage for Intl.NumberFormat.prototype.format by adding
a check for testing if it calls ToNumber (7.1.3) on its argument.

Increase coverage for Intl.NumberFormat by adding a test for format
checking it's default value.

Add tests for Intl.NumberFormat.prototype[@@toStringTag].

Add Intl.NumberFormat.prototype.formatToParts to the test
this-value-not-numberformat.js to check if a TypeError is thrown if the
"this" value for the function call isn't a valid NumberFormat object.