When the signs differ, the precision of the conversion sometimes
drops below 106 bits. This strategy is identical to the
hexadecimal variant.
I've refactored tst-sprintf3 to enable testing a value with more
than 30 significant digits in order to demonstrate this failure
and its solution.
Additionally, this implicitly fixes a typo in the shift
quantities when subtracting from the high mantissa to compute
the difference.
37a4c70bd4