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/filetests/filetests/x64/simple.wat

@@ -0,0 +1,12 @@
+;;! target = "x86_64"
+
+(module
+  (func (result i32)
+    (i32.const 42)
+  )
+)
+;;    0:	 55                   	push	rbp
+;;    1:	 4889e5               	mov	rbp, rsp
+;;    4:	 48c7c02a000000       	mov	rax, 0x2a
+;;    b:	 5d                   	pop	rbp
+;;    c:	 c3                   	ret