test: Simplify elog_add tests

This PR rework a bit the elog_add to simplify it.

• No duplication with the get_cell_output tests
  • The way to retrieve cells is already tested by test_captured_output no need to mix elogbook with that
• No more need prdef and ansi tests
  • There is now tests for standard commands already including tests on cell output
• We only test that the elog_add command interact the way we expect with the blissrepl and the elogbook

FYI @denolf