winch: Adding support for integration tests (#5588)

* Adding in the foundations for Winch `filetests`

This commit adds two new crates into the Winch workspace:
`filetests` and `test-macros`. The intent is to mimic the
structure of Cranelift `filetests`, but in a simpler way.

* Updates to documentation

This commits adds a high level document to outline how to test Winch
through the `winch-tools` utility. It also updates some inline
documentation which gets propagated to the CLI.

* Updating test-macro to use a glob instead of only a flat directory

winch/docs/testing.md

@@ -0,0 +1,63 @@
+# Testing Winch
+
+Winch is tested through integration testing using the `winch-filetests` crate
+and manual exploratory testing. A CLI is available to run these tests
+conveniently. To add the `winch-tools` binary to your `PATH`, run `cargo install
+--path winch` from the root of `wasmtime`. The CLI provides two commands: `test`
+and `compile`. To see the help text for each command, run `winch-tools test
+--help` or `winch-tools compile --help`.
+
+## Integration Testing (`winch-tools test`)
+
+The `test` command will run a suite of tests that validates Winch output for a
+WebAssembly module is consistent with our expectations.
+
+### Running `test`
+
+Running `winch-tools test` will run all integration tests in the
+`winch-filetests` crate. All arguments following two dashes (`--`) will be
+passed directly to `cargo test -p winch-filetests`. This will allow you to
+configure the tests to run based on your requirements. All tests in the
+`winch-filetests` crate get named in the following convention:
+`winch_filetests_${filepath}`. This makes it possible to filter if you don't
+want to run the entire suite.
+
+If the output of Winch changes for a test in a run due to code updates, the test
+will fail and the difference between the two outputs will be shown. If the new
+output is expected, the tests can be re-run with an `WINCH_TEST_BLESS`
+environment variable set to `1`.
+
+### Adding a test
+
+To add new tests, create a `.wat` file in the `winch/filetests/filetests` folder
+in the following format:
+
+```wat
+;;! target = "x86_64"
+(module
+  (func (result i32)
+    (i32.const 42)
+  )
+)
+```
+
+It is encouraged to use folders to organize tests. For example, tests targeting
+the x86_64 architecture can be placed in the `winch/filetests/filetests/x64`.
+
+The first block of comments are a TOML compatible configuration passed to Winch
+during compilation with a `!` at the start of each line. The body of the file
+will be the subject of the test. A final block of comments is reserved for the
+output of the compilation, and it will be used to compare the output of the
+current run with the output of previous runs.
+
+## Manual Exploratory Tests (`winch-tools compile`)
+
+The `compile` command will run Winch for particular architecture against
+provided input file, and print the disassembled output to the console. Only
+`.wat` files are supported.
+
+### Running `compile`
+
+```bash
+winch-tools compile $wat_file --target $target_triple
+```