Add legacy test conversion script.
Merge pull request #1287 '[RFC] Add legacy test conversion script.'
legacy tests: improvements to functional helpers.lua

- Clean vim registers and functions before each test
- Add eval workaround to the request helper method
- Export dedent method
legacy tests: migrate test4
legacy tests: migrate test102
legacy tests: migrate test9
legacy tests: migrate test22
legacy tests: migrate test106
legacy tests: migrate test18
legacy tests: migrate test_options
legacy tests: migrate test_insertcount
legacy tests: migrate test_utf8
legacy tests: migrate test81
legacy tests: migrate test_changelist
legacy tests: migrate test_autoformat_join
legacy tests: migrate test56
legacy tests: migrate test23
legacy tests: migrate test41
legacy tests: migrate test54
legacy tests: migrate test27