refactor(getchar): rename test variable (#34769)

Also, test_disable_char_avail() is superseded by test_override() in Vim,
so remove that from vim_diff.txt.

runtime/doc/vim_diff.txt

@@ -872,9 +872,8 @@ Startup:
 Test functions:
 - test_alloc_fail()
 - test_autochdir()
-- test_disable_char_avail()
 - test_feedinput()
-- test_garbagecollect_soon
+- test_garbagecollect_soon()
 - test_getvalue()
 - test_ignore_error()
 - test_null_blob()
@@ -892,6 +891,8 @@ Test functions:
 - test_setmouse()
 - test_settime()
 - test_srand_seed()
+- test_unknown()
+- test_void()
 
 TUI:
                           *t_xx* *termcap-options* *t_AB* *t_Sb* *t_vb* *t_SI*