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/src/filetests.rs

@@ -0,0 +1,25 @@
+use std::process::Command;
+
+use anyhow::Result;
+use clap::Parser;
+
+#[derive(Parser, Debug)]
+pub struct Options {
+    /// Passes extra arguments to `cargo test --package winch-filetests`. For example, to run a single
+    /// test, use `-- --test-threads 1 --test single_test_name`.
+    #[clap(last = true, value_parser)]
+    cargo_test_args: Vec<String>,
+}
+
+pub fn run(opts: &Options) -> Result<()> {
+    Command::new("cargo")
+        .arg("test")
+        .arg("--package")
+        .arg("winch-filetests")
+        .arg("--")
+        .args(&opts.cargo_test_args)
+        .spawn()?
+        .wait()
+        .map(|_| ())
+        .map_err(|e| anyhow::anyhow!("Failed to run cargo test: {}", e))
+}