Problem:    Various pieces of code not covered by tests.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes #8255)

Problem:    Check for features implemented with "if".
Solution:   Use the Check commands. (Ken Takata, closes #7383)

Problem:    Vim9: trinary operator condition is too permissive.
Solution:   Use tv_get_bool_chk().

Problem:    No falsy Coalescing operator.
Solution:   Add the "??" operator.  Fix mistake with function argument count.

Problem:    Cannot put space between function name and paren.
Solution:   Allow this for backwards compatibility.

Problem:    Error for white space after expression in assignment.
Solution:   Skip over white space. (closes #6617)

Problem:    assert_fails() checks the last error message.
Solution:   Check the first error, it is more relevant.  Fix all the tests
            that rely on the old behavior.

Problem:    Expression "!expr->func()" does not work.
Solution:   Apply plus and minus earlier. (closes #6348)

Problem:    Insufficient testing for invalid function arguments.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5988)

Problem:    Crash with null partial and blob.
Solution:   Check for NULL pointer.  Add more tests. (Yegappan Lakshmanan,
            closes #5984)