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/pr2303.wat

@@ -0,0 +1,15 @@
+(module
+    (memory (export "mem") 1 1)
+    (func (export "runif") (param $cond i32)
+      i32.const 48
+      (v128.load (i32.const 0))
+      (v128.load (i32.const 16))
+      (if (param v128) (param v128) (result v128 v128)
+          (local.get $cond)
+          (then i64x2.add
+                (v128.load (i32.const 32)))
+          (else i32x4.sub
+                (v128.load (i32.const 0))))
+      i16x8.mul
+      v128.store)
+)