testfloat does not generate pseudo-subnormals for extF80

[GregoryMorse]

This cross-references to https://github.com/ucb-bar/berkeley-softfloat-3/issues/37 because as softfloat does not support these values, adding them to test generation would of course fail to validate, but if their support is added in softfloat, they should likewise be generated by testfloat.