The default formatting mimicks printf with %.*p*g where *p* is
the precision to use. It turns out that JavaScript's toPrecision()
method is almost but not exactly equal to %g; they differ for values
with small absolute values (10^-1 to 10^-5 or so), with toPrecision()
yielding strings that are longer than they should be (i.e. using fixed
point where %g would use exponential).

This test is intended to check that our formatting works properly for a variety of precisions.

Precision to use (1 to 21):