You can find different types of Writer tests here, the most interesting ones:

- core/filters-test: loads different file types (see SwFiltersTest::testCVEs()
  for the list of supported filter types), and only makes sure Writer does not
  crash. To add new tests, you just need to drop in the test files to the
  relevant directory, no code change is necessary.
- core/uwriter: this test has access to private Writer symbols, add new tests
  here if you need that.
- extras: see its own README -- executes import and export filters and asserts
  the UNO model to verify the resulting document is the expected one.