Refactoring exception test.

Change to use assert_equal explicitly.
Merge pull request #2191 from yui-knk/refactor-exc-test

Refactoring exception test.
add clang to travis

Enable UB sanitizer, but not the alignment one, because
`src/codegen.c` and therefore `mrbc` print a lot of errors.

Of course, these issues should be fixed in the future.