[lldb] Correct wording of EXP_MSG

EXP_MSG generates a message to show on assert
failure. Currently it looks like:
AssertionError: False is not True : '<cmd>'
returns expected result, got '<actual output>'

Which seems to say that the test failed but
also got the expected result.

It should say:
AssertionError: False is not True : '<cmd>'
returned unexpected result, got '<actual output>'

Reviewed By: teemperor, #lldb

Differential Revision: https://reviews.llvm.org/D86603