cranelift: move wasmtests in cranelift-wasm

Move test data used by cranelift-wasm's tests in
the crate directory, to make the tests autonomous.

Fixes #2910

cranelift/wasm/wasmtests/select.wat

@@ -0,0 +1,19 @@
+(module
+  (func $untyped-select (result i32)
+  	i32.const 42
+  	i32.const 24
+  	i32.const 1
+  	select)
+
+  (func $typed-select-1 (result externref)
+  	ref.null extern
+  	ref.null extern
+  	i32.const 1
+  	select (result externref))
+
+  (func $typed-select-2 (param externref) (result externref)
+    ref.null extern
+    local.get 0
+    i32.const 1
+    select (result externref))
+)