Changes

ruff.toml: new file

Add a file in the root dir of the repository to allow running
"ruff format" in order to auto-format the code with expected max line
length, PEP-8, etc.

Replace _testenv/pyproject.toml with .ruff.toml in the root directory of
the repository, so we can exclude "compare-results.py" which doesn't
follow that code style. Otherwise it would get formatted too when
running "ruff format" in the root dir of the repository.

Change-Id: I97ed1ececba85008ca754cf91bf46522e168a894

Cosmetic: testenv: fix line length

Run "ruff format", so there isn't a diff anymore when running it on
future patches.

Change-Id: I766a07c6d54d0da11ad47acce1fc07b4b3a4801b

testenv: improve --config argument parsing

* Support using wildcards for the config names via fnmatch as that makes
  it much easier to run the ggsn tests against all osmo-ggsn config
  variations, and update the examples in "testenv.py -h" to illustrate
  this.
* Fix that it didn't complain about an invalid --config argument, as
  long as there was a valid --config argument before it.
* Let raise_error_config_arg only output the invalid --config argument
  instead of all of them.
* Complain if "--config all" is used in combination with another
  --config argument.
* Sort testenv*.cfg files found alphabetically, so they are always
  executed in the same order.

Change-Id: I66b976b0332be523c084a6b5d38d0f62134b495d

ggsn: testenv: fix paths in copy= and clean=

I had moved osmo-ggsn related files to the osmo-ggsn directory and
forgot to adjust testenv_osmo_ggsn_{v4,v6,v4v6}_only.cfg. Fix it by
changing it like it is in testenv_osmo_ggsn_all.cfg

Change-Id: Ibc8aeb43ff594a6b26699f81b29abc31b9b5b71f

ggsn: testenv: add libosmocore-utils to package=

Add libosmocore-utils, so osmo-config-merge is installed when running
with --binary-repo too. The osmo-config-merge program is used in
osmo-ggsn/testenv.sh to merge the configs.

Change-Id: I7d97e0f923d928e9b51ff41d50b28706e0e6790b