Proposed test improvement: add test seq.GetEnumerator

seq expressions dont generate check for access of Enumerator.Current before call to Enumerator.MoveNext()

```F#
> ( seq { yield 1; yield 2; } ).GetEnumerator().Current;;
val it : int = 0
```

```F#
> ( seq { for i in 1 .. 10 do yield i } ).GetEnumerator().Current;;
val it : int = 0
```

with range is ok:

```F#
> ( seq { 1 .. 10 } ).GetEnumerator().Current;;
System.InvalidOperationException: Enumeration has not started. Call MoveNext.
```

this pr add test for IEnumerable and IEnumerable< ' T > similar to ListType IEnumerable.
Enumerator.Reset() is not supported for sequence.