internal/mcp: add missing testenv.NeedsExec

Unbreak our WASM tests by correctly marking a test as testenv.NeedsExec.

Change-Id: Ib976657e9b09acc6fc563f6fc26d6066fd64acf5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/678035
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/cmd_test.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/modelcontextprotocol/go-sdk/mcp"
+	"golang.org/x/tools/internal/testenv"
 )
 
 const runAsServer = "_MCP_RUN_AS_SERVER"
@@ -38,6 +39,8 @@ func runServer() {
 }
 
 func TestCmdTransport(t *testing.T) {
+	testenv.NeedsExec(t)
+
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()