Fix output of Prism location test errors

[egiurleo]

Motivation

When I implemented the Prism location test runner, I accidentally switched the diff output so the expected parse tree showed up as the test output and vice versa.

For example, if I purposely make the alias location tests error by removing the symbol name, this is what the output looks like:

       "type" : "Alias",
       "from" : {
         "type" : "Symbol",
-        "val" : ""
+        "val" : "new_method1"
       },

This PR just fixes the test output, making it correct and easier for us to read.

Test plan

See included automated tests.

[st0012]

Thx!