Launch Windows diff tools from inside WSL

[SimonCropp]

Stacked on #947, which stops WSL counting as a build server. Without that, nothing is launched inside WSL at all.

Inside a WSL distribution, a tool with no copy in the distribution is now resolved from its Windows definition and started through WSL's interop. Both files are passed as Windows paths: C:\... on a mounted drive, \wsl.localhost\<distro>\... otherwise.

How

• Resolution (OsSettingsResolver, WslHost): the Linux definition first, then the Windows one. The host's variables come from one cmd.exe /u /c set, its drives from /proc/mounts, the share name from one wslpath -w /. A program on the PATH is found from one listing of each host directory, since a stat on a Windows drive from inside WSL is most of a millisecond.
• Paths (WslPaths): translated in process.
• Launch (WslInterop.Start): all three streams redirected and closed, so the tool does not hold the pipe dotnet test reads.
• Tracking: ps lists such a tool as /init <exe> <exe> <args>, which StripProxy brings back to the command it was started with.

Behaviour to know

• A Windows tool is never closed from WSL: not when its test passes, and a tool that does not auto refresh gets a second window on a repeat failure. Ending the Linux-side process does not close a windowed tool.
• Tool order now reaches Windows tools, so Beyond Compare opens ahead of VS Code where both are installed, as on Windows.
• Not offered: the viewer, a tool whose Windows executable is a .cmd, and Vim and Neovim.
• DiffEngine_WslWindowsTools=false turns it off.
• Tool resolution under WSL went from about 0.24 s to 0.42 s per test process on the machine it was measured on.

Testing

• 63 new tests, which need no WSL: a temp directory stands in for the drive. They pass on .NET 10 and .NET Framework 4.8 on Windows, and inside an Ubuntu distribution.
• By hand in Ubuntu under WSL 2: a failing Verify test opened Beyond Compare, WinMerge and P4Merge with the right files, a second failing run found the open Beyond Compare window, and the run returned with the window still open.
• Not tested: Rider, Guiffy, Diffinity and the Word and Excel comparers (they resolve but were not launched), WSL 1, mirrored networking, and a distribution with the Windows PATH not appended.