Problem:    Vim9: some assignment tests in the wrong file.
Solution:   Move assignment tests to test_vim9_assign.

Problem:    Vim9: Some tests are still using :let.
Solution:   Change the last few declarations to use :var.

Problem:    Vim9: :let will soon be disallowed.
Solution:   Add v:disallow_let temporarily.  Fix tests.

Problem:    Vim9: Cannot use "fina" for "finally". (Naruhiko Nishino)
Solution:   Specifically check for "fina". (closes #7020)

Problem:    Vim9: using ":const!" is weird.
Solution:   Use "var" - "final" - "const" like Dart.  "let" still works for
            now.

Problem:    Test still fails without the terminal feature.
Solution:   Put check for terminal feature in separate function.

Problem:    Test fails without the terminal feature.
Solution:   Skip test if the terminal feature is not available.

Problem:    Vim9: crash when compiling a manually defined function. (Antony
            Scriven)
Solution:   Check that the script ID is positive. (closes #7012)

Problem:    Vim9: assignment tests spread out.
Solution:   Create new test file for assignment tests.

Problem:    Vim9: memory leak with heredoc that isn't executed. (Dominique
            Pellé)
Solution:   Don't clear the list items. (closes #6991)