test: add a test for wasm memory usage

Test that a small Wasm program uses 8 MB of linear memory. This
reflects the current allocator. We test an exact value, but if the
allocator changes, we can update or relax this.

Updates #69018.

Change-Id: Ifc0bb420af008bd30cde4745b3efde3ce091b683
Reviewed-on: https://go-review.googlesource.com/c/go/+/622378
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

test/wasmmemsize.dir/asm_wasm.s

@@ -0,0 +1,11 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+TEXT ·currentMemory(SB), NOSPLIT, $0
+	Get SP
+	CurrentMemory
+	I32Store ret+0(FP)
+	RET

test/wasmmemsize.dir/main.go

@@ -0,0 +1,30 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"io"
+)
+
+// Expect 8 MB of memory usage for a small wasm program.
+// This reflects the current allocator. We test an exact
+// value here, but if the allocator changes, we can update
+// or relax this.
+const want = 8 << 20
+
+var w = io.Discard
+
+func main() {
+	fmt.Fprintln(w, "hello world")
+
+	const pageSize = 64 * 1024
+	sz := uintptr(currentMemory()) * pageSize
+	if sz != want {
+		fmt.Printf("FAIL: unexpected memory size %d, want %d\n", sz, want)
+	}
+}
+
+func currentMemory() int32 // implemented in assembly

test/wasmmemsize.go

@@ -0,0 +1,11 @@
+// runindir
+
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This test checks the memory size of a small wasm program.
+
+//go:build wasm
+
+package ignored