PVF: Add test instructions (#2058)

polkadot/node/core/pvf/README.md

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+# PVF Host
+
+This is the PVF host, responsible for responding to requests from Candidate
+Validation and spawning worker tasks to fulfill those requests.
+
+See also:
+
+- for more information: [the Implementer's Guide][impl-guide]
+- for an explanation of terminology: [the Glossary][glossary]
+
+## Running basic tests
+
+Running `cargo test` in the `pvf/` directory will run unit and integration
+tests.
+
+**Note:** some tests run only under Linux, amd64, and/or with the
+`ci-only-tests` feature enabled.
+
+See the general [Testing][testing] instructions for more information on
+**running tests** and **observing logs**.
+
+## Running a test-network with zombienet
+
+Since this crate is consensus-critical, for major changes it is highly
+recommended to run a test-network. See the "Behavior tests" section of the
+[Testing][testing] docs for full instructions.
+
+To run the PVF-specific zombienet test:
+
+```sh
+RUST_LOG=parachain::pvf=trace zombienet --provider=native spawn zombienet_tests/functional/0001-parachains-pvf.toml
+```
+
+## Testing on Linux
+
+Some of the PVF functionality, especially related to security, is Linux-only,
+and some is amd64-only. If you touch anything security-related, make sure to
+test on Linux amd64! If you're on a Mac, you can either run a VM or you can hire
+a VPS and use the open-source tool [EternalTerminal][et] to connect to it.[^et]
+
+[^et]: Unlike ssh, ET preserves your session across disconnects, and unlike
+another popular persistent shell, mosh, it allows scrollback.
+
+[impl-guide]: https://paritytech.github.io/polkadot-sdk/book/pvf-prechecking.html#summary
+[glossary]: https://paritytech.github.io/polkadot-sdk/book/glossary.html
+[testing]: https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/doc/testing.md
+[et]: https://github.com/MisterTea/EternalTerminal