Test that internal constructor return type asserts are respected (Jul…

…iaLang#47227)

On Julia 1.8, if the internal constructor's return value was typeasserted,
this assertion could be violated without an error. For example, in this case:

```julia
struct Foo
    x::Int
    Foo()::Nothing = new(1)
end
```

Running `Foo()` would not throw. This was inadvertently fixed in a later PR,
but not tested.
This PR adds a test case similar to the example above.

test/core.jl

@@ -4859,6 +4859,13 @@ let a = Any[]
  @test a == [10, 2]
 end
 
+# issue 47209
+struct f47209
+ x::Int
+ f47209()::Nothing = new(1)
+end
+@test_throws MethodError f47209()
+
 # issue #12096
 let a = Val{Val{TypeVar(:_, Int)}},
  b = Val{Val{x} where x<:Int}