Update eval_test.go

Added new test for multiline raw strings (currently failing).
Fixed bug in func check() where expected and actual results were swapped in printed error statement.
Made the printed error statements for expected out and err more consistent.
Trimmed whitespace after newline (by running eval_test.go through gofmt).
Merge pull request #6 from shurcooL/patch-1

Update eval_test.go