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/if-reachability-translation-4.wat

@@ -0,0 +1,12 @@
+;; Reachable `if` head and unreachable consequent and alternative means that the
+;; following block is unreachable.
+
+(module
+  (func (param i32) (result i32)
+    local.get 0
+    if
+      unreachable
+    else
+      unreachable
+    end
+    i32.const 0))