Split out testing modules and test bzlmod mode on CI.

[aherrmann]

Part of #181.

As outlined in tweag/rules_sh#41 advanced test-setups within a Bazel module are problematic when publishing the module on the Bazel Central Registry. To that end test modules should be split out into their own modules.

We then want to make sure that the Bazel modules themselves and their testing modules can be built and tested successfully in both workspace and bzlmod mode on CI.

• Separate core tests and test with bzlmod enabled #319
• Separate toolchains/java tests and run with bzlmod enabled #320
• Separate testing module and bzlmod tests for toolchains/cc #325
• Separate testing module and bzlmod tests for toolchains/go #326
• Separate testing module and bzlmod tests for toolchains/nodejs #332
• Separate testing module and bzlmod tests for toolchains/posix #323
• Separate testing module and bzlmod tests for toolchains/python #335
• Separate testing module and bzlmod tests for toolchains/rust #329

[aherrmann]

All testing modules were split out and all modules are tested on CI with bzlmod enabled.