test: reproduce bad inference behavior of @mel.as

[anmonteiro]

we made a mistake when inferring @mel.as in external polyvariants in #722.

@mel.string actually meant @mel.spread, which we'll be making explicit in #1404.

inferring @mel.as should not splice in the arguments as shown by the reproduction in the test case added in this PR.

[anmonteiro]

@mel.as here should just have changed the argument to "bar":

foo({
  NAME: "foo",
  VAL: 2
});