test: cherry-pick improvements from master branch

Partial cherry-pick of b3686b1597.

* test/functional/provider: display reason from missing_provider
* provider#node#Detect: skip / handle non-existing node executable

runtime/autoload/provider/node.vim

@@ -51,6 +51,9 @@ function! provider#node#Detect() abort
   if exists('g:node_host_prog')
     return expand(g:node_host_prog)
   endif
+  if !executable('node')
+    return ''
+  endif
   if !s:is_minimum_version(v:null, 6, 0)
     return ''
   endif

test/functional/provider/nodejs_spec.lua

@@ -8,8 +8,9 @@ local retry = helpers.retry
 
 do
   clear()
-  if missing_provider('node') then
+  local reason = missing_provider('node')
-    pending("Missing nodejs host, or nodejs version is too old.", function()end)
+  if reason then
+    pending(string.format("Missing nodejs host, or nodejs version is too old (%s)", reason), function() end)
     return
   end
 end

test/functional/provider/python3_spec.lua

@@ -10,13 +10,14 @@ local pcall_err = helpers.pcall_err
 
 do
   clear()
-  if missing_provider('python3') then
+  local reason = missing_provider('python3')
+  if reason then
     it(':python3 reports E319 if provider is missing', function()
       local expected = [[Vim%(py3.*%):E319: No "python3" provider found.*]]
       matches(expected, pcall_err(command, 'py3 print("foo")'))
       matches(expected, pcall_err(command, 'py3file foo'))
     end)
-    pending('Python 3 (or the pynvim module) is broken/missing', function() end)
+    pending(string.format('Python 3 (or the pynvim module) is broken/missing (%s)', reason), function() end)
     return
   end
 end

test/functional/provider/python_spec.lua

@@ -18,13 +18,14 @@ local pcall_err = helpers.pcall_err
 
 do
   clear()
-  if missing_provider('python') then
+  local reason = missing_provider('python')
+  if reason then
     it(':python reports E319 if provider is missing', function()
       local expected = [[Vim%(py.*%):E319: No "python" provider found.*]]
       matches(expected, pcall_err(command, 'py print("foo")'))
       matches(expected, pcall_err(command, 'pyfile foo'))
     end)
-    pending('Python 2 (or the pynvim module) is broken/missing', function() end)
+    pending(string.format('Python 2 (or the pynvim module) is broken/missing (%s)', reason), function() end)
     return
   end
 end

test/functional/provider/ruby_spec.lua

@@ -18,13 +18,14 @@ local pcall_err = helpers.pcall_err
 
 do
   clear()
-  if missing_provider('ruby') then
+  local reason = missing_provider('ruby')
+  if reason then
     it(':ruby reports E319 if provider is missing', function()
       local expected = [[Vim%(ruby.*%):E319: No "ruby" provider found.*]]
       matches(expected, pcall_err(command, 'ruby puts "foo"'))
       matches(expected, pcall_err(command, 'rubyfile foo'))
     end)
-    pending("Missing neovim RubyGem.", function() end)
+    pending(string.format('Missing neovim RubyGem (%s)', reason), function() end)
     return
   end
 end