Previously we would calculate the index of the first parent node as
heap.size() (which is initialized to non_zero_freqs), so in the edge
case in which all symbols had a non-zero frequency, we would use the
Size-index entry in the array for both the first symbol's leaf node,
and the first parent node.
The result would either be a non-optimal huffman code (bad), or an
illegal huffman code that would then go on to crash due to an error
check in CanonicalCode::from_bytes. (worse)
We now store parent nodes starting at heap.size() - 1, which eliminates
the potential overlap, and resolves the issue.
Using a file(GLOB) to find all the test files in a directory is an easy
hack to get things started, but has some drawbacks. Namely, if you add
a test, it won't be found again without re-running CMake. `ninja` seems
to do this automatically, but it would be nice to one day stop seeing it
rechecking our globbed directories.
