Merge wp_networking crate into wp_api

[oguzkocer]

Builds on #99.

wp_networking was initially separated, so we could the wp_api as a dependency crate, rather than an internal one. Later on wp_networking became an integration test suite and in Rust, integration tests already treat the crate as a dependency, so there is no benefit left in keeping them separate.

The PR merges wp_networking into wp_api by:

• Moving wp_networking dependencies to wp_api's dev-dependencies.
• Moving the tests folder as a whole to wp_api.
• Moving the contents of wp_networking/src/lib.rs to tests/test_helpers. Note the only implementation here was AsyncWPNetworking which for now is only needed for integration tests. I think one day we might add a default client to wp_api itself, but for now this should be enough.

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As a bonus, I've renamed the src/test_helpers to src/unit_test_common in 1c0f443 and tests/test_helpers to tests/integration_test_common in 8da085a. Both of these modules having the same name was very annoying when browsing the code and I'd like to move away from the helper suffix as much as possible, so I picked these for now. Happy to change it again if you have a suggestion for a better name.

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To Test

• make test-server && make dump-mysql && make backup-wp-content-plugins
• cargo test --test '*' -- --nocapture --test-threads 1