[analyzer] [NFC] Reverse the argument order for "diff" in tests

The current argument order has "expected" and "actual" the wrong way around,
so that the diff shows the change from expected to actual, not from actual to expected.

Namely, if the expected diagnostics contains the string "foo", but the analyzer emits "bar",
we really want to see:

```
- foo
+ bar
```

not

```
- bar
+ foo
```

since adapting to most changes would require applying that diff to the expected output.

Differential Revision: https://reviews.llvm.org/D56340

llvm-svn: 350866