Floating-point values not printed with enough precision

I recently had this failure on a CI platform:
```
C:/projects/arrow/cpp/src/arrow/util/decimal_test.cc(557): error: Expected equality of these values:
dec.ToReal(scale)
Which is: 1e-163
expected
Which is: 1e-163
```

Needless to say, this is not very informative :-) Floating-point values should be printed with enough precision to recreate the exact same values from the string representation, so as to diagnose issues.