Refactor the test suite

- Remove superfluous test cases that did not test anything new.
- Rename test files to match the names of the tested checkers.
- Rename the resource files to match the name of the tests the belong
  to.
- Do not re-use the same resource files for different test files.  Make
  sure that each test file gets its own resources.

This refactoring eases orientation within the test suite, and makes the
whole thing easier to comprehend and understand.
Fix ordering in syntax checker index