Tweak invalid construction policy tests

[chiphogg]

The "complex to real" test was based on some assumptions about how
std::complex worked which I had never verified. It turns out that
whether or not we permit certain conversions, they may not even be
possible. For example: you can't actually construct a
std::complex<float> from a std::complex<int>, even though you can of
course construct a float from an int!

Therefore, we delete all irrelevant aspects of the test case, and focus
it down to its core: that std::complex<float> to float is forbidden.

The "negative ratio, unsigned destination" test will begin to fail for
an unrelated reason, with our new and more carefully designed
construction policy. uint8_t has a max value of 255, and int8_t a
max value of 127. We will now prevent assigning the former to the
latter because the risk of overflow is too high. (The max
non-overflowing value is 127, which is smaller than our threshold of
2147.)

Here, the fix is to just use 16-bit integers.

Helps #349.