patch 8.2.4684: cannot open a channel on a Unix domain socket

Problem:    Cannot open a channel on a Unix domain socket.
Solution:   Add Unix domain socket support. (closes #10062)

src/testdir/shared.vim

@@ -15,10 +15,16 @@ func PythonProg()
   if has('unix')
     " We also need the job feature or the pkill command to make sure the server
     " can be stopped.
-    if !(executable('python') && (has('job') || executable('pkill')))
+    if !(has('job') || executable('pkill'))
       return ''
     endif
-    let s:python = 'python'
+    if executable('python')
+      let s:python = 'python'
+    elseif executable('python3')
+      let s:python = 'python3'
+    else
+      return ''
+    end
   elseif has('win32')
     " Use Python Launcher for Windows (py.exe) if available.
     " NOTE: if you get a "Python was not found" error, disable the Python