Switch expected and actual in assert_eq!

The original XUnit was expected, actual, but we had actual, expected.
http://junit.sourceforge.net/javadoc/org/junit/Assert.html#assertEquals%28double,%20double%29

With the new formatting, this doesn't matter as much, but it's nice to be
consistent with previous frameworks, and if we change the output such
that it does matter, better to be ready.
auto merge of #6847 : steveklabnik/rust/switch_expected_given, r=brson

The original XUnit was expected, actual, but we had actual, expected.
http://junit.sourceforge.net/javadoc/org/junit/Assert.html#assertEquals%28double,%20double%29

With the new formatting, this doesn't matter as much, but it's nice to be
consistent with previous frameworks, and if we change the output such
that it does matter, better to be ready.