atests: Enhancing 'Should Contain Tests'

- Enhanced suppport for passing custom status.
  This eases creating tests for --merge.
- Case-sensitive test name matching.
- Removed 'Check Suite Contains Tests' alias.

Last two changes required quite a few changes to tests.
Rebot: New generic --merge functionality (#

- Similar to old --rerunmerge which it replaces.
- Main difference is that new suites/tests in merged outputs are added
  into the original suite structure. Earlier they caused an error.
- Having different root suites is still an error, nowadays actually
  a fatal one failing the whole post-processing.
- Suite start/end times are cleared if tests/suites added to it.
  Causes elapsed time to be re-calculated. Times would be correct anyway.

TODO:
- Decided how to add a note that tests/suites have been added/replaced.
  Currently test/suite message is updated, but could also use documentation
  and with suits also Metadata.
- Documentation in User Guide.
Small enhancements to --help