rumur-run: avoid using file system if possible

[Smattr]

Sometimes you have no access to the file system (nothing mounted, you're chrooted, ro mounts, ...). It would be nice if rumur-run still worked in such cases. This looks achievable on Linux using memfd_create and fexecve. I don't know how we can achieve this on other platforms, but Linux may be the only one where this situation regularly arises.

[Smattr]

This is somewhat harder than expected because memfd_create was only exposed in Python in 3.8. For the fexecve part, it looks like we can simply call os.execve with a file descriptor.

[Smattr]

We could also do some of this in the test suite, to use pipes instead of disk where possible.

[Smattr]

The rewritten test suite does this about as extensively as possible without resorting to the memfd_create tricks described above.