`@isdefined` sometimes returns a non-boolean

```julia
using Test: Error, @testset

f() = rand(3,3)

@testset "a" begin
    r0 = f()

    (@isdefined(Error))::Bool
    for x in ()
    end
end
```

errors with

```
  TypeError: non-boolean (DataType) used in boolean context
```

The returned value seems to be the `Error` itself. Doing random stuff like commenting out the loop or the function call or the test set makes the error go away.