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-6.wat

@@ -0,0 +1,14 @@
+;; Reachable `if` head and unreachable consequent and alternative, but with a
+;; branch out of the alternative, means that the following block is reachable.
+
+(module
+  (func (param i32 i32) (result i32)
+    local.get 0
+    if
+      unreachable
+    else
+      local.get 1
+      br_if 0
+      unreachable
+    end
+    i32.const 0))