Test.only should be multiplicative instead of additive

```elm
suite1 =
Test.only <|
Test.describe "foo"
[ Test.test "Abc" <| \() -> Expect.pass
, Test.only <|
Test.test "Def" <| \() -> Expect.pass
]

suite2 =
Test.describe "bar"
[ Test.test "Ghi" <| \() -> Expect.pass
]
```

This should result in only `Def` being run, but it results in both `Abc` and `Def` being run, because `Test.only` in a sense uses set union instead of set intersection.

Start by reproducing the nested Test.only example from the issue and compare the runs from suite1 and suite2. Inspect the implementation of Test.only and its handling of nested test selections; done means only Def runs when the outer and inner selections are combined, with coverage for this example.

Written by the indexing model from the issue text.