internal/core/adt: test TopKind in TestKindString

It has special logic to stringify as "_", so it seems odd to omit it
from the table-driven unit test.

Signed-off-by: Daniel Martí <[e-mail]>
Change-Id: I8144ea52d1e8c315617cce730a32fa37d0747093
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1224222
Reviewed-by: Roger Peppe <[e-mail]>
TryBot-Result: CUEcueckoo <[e-mail]>
Unity-Result: CUE porcuepine <[e-mail]>

internal/core/adt/kind_test.go

···
 		input: BottomKind,
 		want:  "_|_",
 	}, {
+		input:     TopKind,
+		want:      "_",
+		wantKinds: []Kind{NullKind, BoolKind, IntKind, FloatKind, StringKind, BytesKind, FuncKind, ListKind, StructKind},
+	}, {
 		input:     IntKind | ListKind,
 		want:      `(int|[...])`,
 		wantKinds: []Kind{IntKind, ListKind},