Clean up the integration tests

[Michael-F-Bryan]

This PR does a couple things to make our CLI integration test story a bit nicer:

• The run-unstable integration tests have been merged into run.rs because wasmer run-unstable no longer exists
• We now use the --registry flag from Added a WasmerEnv abstraction to the CLI #4013 instead of running wasmer login and clobbering the tokens in the user's $WASMER_DIR/wasmer.toml (Fixes The wasmer-integration-tests-cli test suite isn't hygienic #4015)
• The run-unstable tests now run the wasmer executable directly instead of making a defensive cargo run --features=...
  • The cargo run doesn't make sense any more because Make CLI integration tests automatically run the most up-to-date version of the CLI #4020 decided that we'll rely on make integration-test-cli or the user to provide an up-to-date wasmer executable
• Some create-exe tests were moved from run.rs into create_exe.rs where they belong
• Moved the paths for all files used during testing into a single fixtures module
• Remove the --test-threads=1 from make test-integration-cli so we use all cores when running integration tests
• Remove the --no-capture from make test-integration-cli to reduce the amount of spam you are bombarded with when running tests

After these changes, the "Test Integration CLI" CI step is reduced from taking about 30 minutes to 3 minutes.

It also looks like the MUSL segfaults are gone (i.e. fixes #3698).

[Michael-F-Bryan]

I'm going to put this PR on hold until #4050 is merged.

We're seeing a bunch of new runtime-related issues in the snapshot tests, which doesn't really make sense this PR hasn't touched any code in the main wasmer-wasix crate. Hopefully they're just spurious failures due to non-determinism and bugs, and the epoll PR will fix things 🤞