For some time now (I'd guess almost a year 🤷), the coverage
merging on Windows has been failing, because the reports have been
generated in a different folder than expected. Our merge script did
not report failure because it was not checking the returned error
code from OpenCppCoverage, and for some reason, the `codecov` tool
happily returned 0 even though it did not find the file it was
supposed to upload...
The former is also fixed by this commit.
Also had to add new project to redirect CTest output, add
separate batch scripts for AppVeyor because it doesn't handle
multi-line batch scripts in yaml properly, and other helper
scripts.
