core: Fix TestContext2Apply_resourceCountOneList

We've intentionally changed the behavior for "count = 1" so that it'll
assign an index to the created instance even though there's only one. The
un-indexed behavior now applies only if count isn't set _at all_, thus
avoiding weird behavior if a count is _dynamically_ set to 1 via an
expression but is assumed to be a list elsewhere in configuration.

terraform/context_apply_test.go

@@ -182,7 +182,7 @@ func TestContext2Apply_resourceCountOneList(t *testing.T) {
 	assertNoDiagnostics(t, diags)
 
 	got := strings.TrimSpace(state.String())
-	want := strings.TrimSpace(`null_resource.foo:
+	want := strings.TrimSpace(`null_resource.foo.0:
   ID = foo
   provider = provider.null