Separate testing module and bzlmod tests for toolchains/python

[evertedsphere]

Adds a new Bazel module in testing/python and moves tests for toolchains/python into this module, following the motivation from tweag/rules_sh#41.

The toolchain registration step is also made optional and disabled in bzlmod mode, as it is not allowed to invoke native.register_toolchains outside MODULE.bazel in the latter case.

[evertedsphere]

Tests only fail on macOS, due to some CC toolchain issues:

  ERROR: /private/var/tmp/_bazel_runner/4b8f17f3ba765bf36a9ba20170a1578c/external/_main~non_module_deps~nixpkgs_python_configure_test/BUILD.bazel:1:8: 
While resolving toolchains for target @_main~non_module_deps~nixpkgs_python_configure_test//:python2-test: 
invalid registered toolchain '@nixpkgs_config_cc_toolchains//:cc-toolchain-k8': 
no such target '@_main~non_module_deps~nixpkgs_config_cc_toolchains//:cc-toolchain-k8': 
target 'cc-toolchain-k8' not declared in package '' defined by /private/var/tmp/_bazel_runner/4b8f17f3ba765bf36a9ba20170a1578c/external/_main~non_module_deps~nixpkgs_config_cc_toolchains/BUILD.bazel (Tip: use `query "@nixpkgs_config_cc_toolchains//:*"` to see all the targets in that package)

[benradf]

Looks good!

[aherrmann]

Thank you!