make expect_stdout node version specific (#1963)

... via semver string on `node_version` label.

test/compress/node_version.js

@@ -0,0 +1,12 @@
+eval_let: {
+    input: {
+        eval("let a;");
+        console.log();
+    }
+    expect: {
+        eval("let a;");
+        console.log();
+    }
+    expect_stdout: ""
+    node_version: ">=6"
+}