My testing is practical rather than prescriptive.
I am using LLVM-MinGW on FreeBSD to cross compile my winlua.net toolchain (https://winlua.net). WinLua includes lua, luarocks, libressl and the llvm-mingw compiler (on windows). I then package that and use it as a C/C++ toolchain on Windows. There is 2 WIndows installations at work and I use it at home on Windows. I have recently used it to cross port netpgp to Windows.
While surely not exhaustive, my use case is quite extensive and could be used to validate that the LLVM-Mingw compiler toolchain works on FreeBSD because it produces a working windows compiler and other software.