Various features to tester

[xeniagda]

Big pull request to add all options and small fixes to the tester I did in my project:

• --args-option to pass extra args to the compiler executable.
• --n-times-option to run the tester multiple times on the same executable.
• Show time taken by the compiler on each test.
• Better support for running the tester on multiple backends in the same run:
  • Show the backend being ran in the status bar
  • Show which backend each error occurs in
  • Fix a bug where the number of total tests is miscalculated when running with multiple backends (and no message is shown when all tests pass)