float.toString() result is lossy

It appears the built-in predicate `float.toString()` causes precision loss for the result. This can be irritating and also prevents using it in QL `InlineExpectationsTest` because there the expected `value` has to be a `string`.

CodeQL's `float` uses 64-bit and is therefore able to represent for example 1.555555555; yet `1.555555555.toString()` has 1.555556 as result.
These are not the same values, `1.555555555 = 1.555556` does not hold.

Start by locating the implementation of the built-in float.toString() predicate and the CodeQL QL InlineExpectationsTest mentioned in the issue. Reproduce the 1.555555555 example and inspect the existing expected string handling; done means the string preserves the float's precision and the relevant expectation passes.

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