Update test to force error under NLL.

Update test to force error under NLL.

In each of the three cases in this test, there is a mutable borrow
of some field of the union and then a shared borrow of some other field
immediately following.

Under NLL, the mutable borrow is killed straight away as it isn't
used later - therefore not causing a conflict with the shared borrow.
This commit adds a use of the first mutable borrow to force the intended
errors to appear under NLL.