Test that default constructed objects are zero

In a generic context, we often need to compare arithmetic types to zero.
However, mp-units types cannot be default constructed from (say) `double`,
so we cann write (e.g.)

```cpp
if (x < T(0)) {
  // ...
}
```

Instead, we have to use

```cpp
constexpr auto ZERO = T{};
if (x < ZERO) {
   // ...
}
```

This begs the question: Do we have a guarantee that `ZERO==0.0`?
Add a unit test that verifies this behavior.

test/unit_test/static/math_test.cpp

@@ -37,9 +37,14 @@ template<typename T1, typename T2, typename... Ts>
 {
   return is_same_v<T1, T2> && v1 == v2 && (... && (v1 == vs));
 }
-
+// Ensure that default constructed objects are zero:
+using T = decltype(2.0 * s);
+constexpr auto ZERO = T{};
+static_assert(ZERO + ZERO == ZERO);
 #if __cpp_lib_constexpr_cmath || MP_UNITS_COMP_GCC
-
+// What if ZERO is not = 0, but infinity?
+// Then the test above still passes. So check that the value is finite:
+static_assert(isfinite(ZERO));
 static_assert(compare(fma(2.0 * s, 3.0 * Hz, 1.0 * one), 7.0 * one));
 static_assert(compare(fma(2.0 * one, 3.0 * m, 1.0 * m), 7.0 * m));
 static_assert(compare(fma(2.0 * m, 3.0 * one, 1.0 * m), 7.0 * m));