Rewrite integration test suite in Python

While the previous test suite ran fine in practice, it relied on
subtle differences in behavior of many UNIX tools, and thus needed GNU
Coreutils on many operating systems to work properly, and didn't work on
"pure" Windows at all.

A simple example would be how different versions of tail handle SIGPIPE
if you pipe its output into another process: GNU tail exits on SIGPIPE
since about 2017 (too lazy to look up the exact version, but this
changed recently), while most other tails continue reading the file and
piping the output into god knows where.

Since we need Python to run the new build system (meson) anyway, let's
use it for the test suite, and get rid of all other test dependencies.

It (currently) requires only the standard library of Python 3.6 or newer.

Unlike the original test suite, this one assigns node names and port
numbers randomly to support `meson test --repeat` (because meson can run
the same test multiple times in parallel, and this breaks with the old
approach).

Also remove old integration tests based on shell scripts.