fix gpu warnings with signed/unsigned char
Merge pull request #1017 from jet47:fix-gpu-warnings
A few minor improvements to the XLS report generator.

* In comparison column headers, switched the order of labels, so that
  it's "to" vs "from".

* When a test was present, but not run successfully, put its status in
  the corresponding cell instead of coloring it gray.
Use log formatting as intended.
Merge pull request #1033 from SpecLad:xls++