run-make: add some basic docs about the test suite's setup

tests/run-make/README.md

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+# The `run-make` test suite
+
+The `run-make` test suite contains tests which are the most flexible out of all
+the [rust-lang/rust](https://github.com/rust-lang/rust) test suites. `run-make`
+tests can basically contain arbitrary code, and are supported by the
+[`run_make_support`] library.
+
+## Infrastructure
+
+There are two kinds of run-make tests:
+
+1. The new `rmake.rs` version: this allows run-make tests to be written in Rust
+   (with `rmake.rs` as the main test file).
+2. The legacy `Makefile` version: this is what run-make tests were written with
+   before support for `rmake.rs` was introduced.
+
+The implementation for collecting and building the `rmake.rs` recipes (or
+`Makefile`s) are in
+[`src/tools/compiletest/src/runtest.rs`](../../src/tools/compiletest/src/runtest.rs),
+in `run_rmake_v2_test` and `run_rmake_legacy_test`.
+
+### Rust-based `run-make` tests: `rmake.rs`
+
+The setup for the `rmake.rs` version is a 3-stage process:
+
+1. First, we build the [`run_make_support`] library in bootstrap as a tool lib.
+2. Then, we compile the `rmake.rs` "recipe" linking the support library and its
+   dependencies in, and provide a bunch of env vars. We setup a directory
+   structure within `build/<target>/test/run-make/`
+
+   ```
+   <test-name>/
+       rmake.exe              # recipe binary
+       rmake_out/             # sources from test sources copied over
+   ```
+
+   and copy non-`rmake.rs` input support files over to `rmake_out/`. The
+   support library is made available as an [*extern prelude*][extern_prelude].
+3. Finally, we run the recipe binary and set `rmake_out/` as the working
+   directory.
+
+[`run_make_support`]: ../../src/tools/run-make-support
+[extern_prelude]: https://doc.rust-lang.org/reference/names/preludes.html#extern-prelude