Replace RunTest.bat and RunGrepTest.bat with Powershell

[NWilson]

Then developers wouldn't need to use windows, since Powershell is available on Linux as well. It's also a sane modern language, compared with .bat, which is virtually impossible to reason about and debug.

I don't think there should be any problem requiring pwsh 5+ to run the tests on windows. It's preinstalled on win 10, and available for older platforms.

[NWilson]

I considered this.

However, Windows PowerShell 5 has some remarkably bad design features: it's very hard to invoke a process and capture its output as un-edited binary data. (Yes, really.) There is a way I found using reflection to invoke .NET APIs to redirect the output to a file; then, use some more .NET to read the file into a buffer without scrambling its bytes. But really, it's obscenely hard.

And even after that, it's obscenely slow.

So the PowerShell file we'd get at the end would be barely any easier to work with (if at all) compared to the Bat file we have. Remarkable, given that it's generally a very nice & modern shell language. It just has pants support for handling binary data (like, run this process and check it generated the correct output bytes).

You know... I'm seriously tempted just to write a tiny POSIX shell for Windows, which would actually be able to interpret the RunTest and RunGrepTest scripts. Is that insane - 1000 lines of C to interpret two 1000-line shell scripts, just to run some testing commands? Maybe. But it could also be fun. In any case, it's not a priority for PCRE2, so I'll put this on the backlog of things that don't really need doing.