Adds method call support for all functions in the patch, but it cannot
be fully ported due to missing tests for:
- filereadable(): requires v8.1.1378 for Test_delete_rf(), but there
appears to have been some trouble porting it. (#12784)
- confirm(): requires v8.1.0832 for Test_confirm() and v8.1.0815 for
feedkeys()'s "L" flag.
(I did attempt to port the test using nvim_input() instead,
but seems that input handling for confirm() doesn't work in
--headless mode?)
Note that confirm() was actually added as a method in
v8.1.1915.
Uncomment use of method call syntax in Test_Executable() previously
included instead from v8.2.2259.
