Add process-utils for testing process execution

[straight-shoota]

process_spec needs some example programs to test the behaviour of Process.run etc.
Previously, it used several platform-specific commands and shell scripts to do that.

This patch introduces a portable implementation of helper programs in Crystal. They are compiled directly into the spec executable and can be called like spec_binary pu cat (this happens implicitly via Process.executable_path).

As a result, the same helper commands work across all platforms and the only requirement is the spec executable. The commands are much cleaner which makes it easier to understand the specs.
We can drop a lot of platform-specific details in the spec code.

I'm leaving print_env_shell_command for now because it's used in a shell: true spec which probably needs a different refactor.

• One spec is broken on MSYS2 due to ENV propagation broken when spawining process with clear_env: true on MSYS2 #17046. I'll probably skip that.
• This doesn't work in the interpreter, of course. I'm considering adding the helper commands as a (hidden) part of the compiler, maybe as an external crystal-pu subcommand?

We've been discussed this idea for a long time, I was finally reminded of it by https://log.pfad.fr/2026/self-calling-executable/ and https://abhinavg.net/2022/05/15/hijack-testmain/