Rename assert_eq arguments to left and right.

The names expected and actual are not used anymore in the output. It also
removes the confusion that the argument order is the opposite of junit.
auto merge of #19647 : nielsegberts/rust/master, r=pnkfelix

The names expected and actual are not used anymore in the output. It also
removes the confusion that the argument order is the opposite of junit.

Bug #7330 is relevant.