ci: test powershell core on Linux

.travis.yml

@@ -115,9 +115,12 @@ jobs:
         apt:
           sources:
             - sourceline: 'ppa:ubuntu-toolchain-r/test'
+            - sourceline: 'deb [arch=amd64] https://packages.microsoft.com/ubuntu/16.04/prod xenial main'
+              key_url: 'https://packages.microsoft.com/keys/microsoft.asc'
           packages:
             - *common-apt-packages
             - gcc-9
+            - powershell
     - if: branch = master AND commit_message !~ /\[skip.lint\]/
       name: lint
       os: linux

test/functional/helpers.lua

@@ -504,9 +504,13 @@ function module.source(code)
   return fname
 end
 
+function module.has_powershell()
+  return module.eval('executable("'..(iswin() and 'powershell' or 'pwsh')..'")') == 1
+end
+
 function module.set_shell_powershell()
   local shell = iswin() and 'powershell' or 'pwsh'
-  assert(module.eval('executable("'..shell..'")'))
+  assert(module.has_powershell())
   local cmd = 'Remove-Item -Force '..table.concat(iswin()
     and {'alias:cat', 'alias:echo', 'alias:sleep'}
     or  {'alias:echo'}, ',')..';'

test/functional/ui/output_spec.lua

@@ -10,6 +10,7 @@ local iswin = helpers.iswin
 local clear = helpers.clear
 local command = helpers.command
 local nvim_dir = helpers.nvim_dir
+local has_powershell = helpers.has_powershell
 local set_shell_powershell = helpers.set_shell_powershell
 
 describe("shell command :!", function()
@@ -228,7 +229,7 @@ describe("shell command :!", function()
       ]])
     end)
   end)
-  if iswin() or eval('executable("pwsh")') == 1 then
+  if has_powershell() then
     it('powershell supports literal strings', function()
       set_shell_powershell()
       local screen = Screen.new(30, 4)