patch 9.2.0574: tests: missing test for v9.2.0572

patch 9.2.0574: tests: missing test for v9.2.0572

Problem:  tests: missing test for v9.2.0572 (the patch accidentally
          included some additional unwanted changes from #20372)
          (thinca)
Solution: Add additional test for delfunc
          (thinca)

Patch 9.2.0572 fixed ":call d.key()" and ":delfunction d.key" failing
with E1017 in Vim9 script, but only the :call form was covered by a
regression test.  Add Test_delfunction_dict_funcref to exercise both
"delfunction d.key" and "delfunction d['k2']".

closes: #20372

Signed-off-by: thinca <thinca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>