Fix 2726 - fix terraform linter.

Instead of using `terraform fmt` for linting use `terraform validate`
with json output.

test/handler/test_terraform_handler.vader

@@ -1,34 +1,61 @@
 Before:
   " Load the file which defines the linter.
   runtime ale_linters/terraform/terraform.vim
+  call ale#test#SetDirectory('/testplugin/test/terraform_files')
+  call ale#test#SetFilename('providers.tf')
 
 After:
   " Unload all linters again.
   call ale#linter#Reset()
+  call ale#test#RestoreDirectory()
 
 Execute(The output should be correct):
   AssertEqual
   \ [
   \   {
-  \     'lnum': 1,
-  \     'col': 20,
-  \     'type': 'E',
-  \     'text': 'illegal char',
+  \     'lnum': 17,
+  \     'col': 13,
+  \     'filename': '/testplugin/test/terraform_files/providers.tf',
+  \     'type': 'W',
+  \     'text': 'Terraform 0.13 and earlier allowed provider version',
   \   },
   \   {
-  \     'lnum': 2,
-  \     'col': 14,
+  \     'lnum': 0,
+  \     'col': 0,
+  \     'filename': '/testplugin/test/terraform_files/providers.tf',
   \     'type': 'E',
-  \     'text': 'literal not terminated',
-  \   },
-  \   {
-  \     'lnum': 1,
-  \     'type': 'E',
-  \     'text': 'object expected closing RBRACE got: EOF',
-  \   },
+  \     'text': 'Plugin reinitialization required. Please run "terraform"',
+  \   }
   \ ],
   \ ale_linters#terraform#terraform#Handle(bufnr(''), [
-  \ 'Error running fmt: In <standard input>: At 1:20: illegal char',
-  \ 'Error running fmt: In <standard input>: At 2:14: literal not terminated',
-  \ 'Error running fmt: In <standard input>: object expected closing RBRACE got: EOF',
+  \ '{',
+  \ '"valid": false,',
+  \ '"error_count": 1,',
+  \ '"warning_count": 1,',
+  \ '"diagnostics": [',
+  \ '  {',
+  \ '    "severity": "warning",',
+  \ '    "summary": "Version constraints inside provider configuration blocks are deprecated",',
+  \ '    "detail": "Terraform 0.13 and earlier allowed provider version",',
+  \ '    "range": {',
+  \ '      "filename": "providers.tf",',
+  \ '      "start": {',
+  \ '        "line": 17,',
+  \ '        "column": 13,',
+  \ '        "byte": 669',
+  \ '      },',
+  \ '      "end": {',
+  \ '        "line": 17,',
+  \ '        "column": 24,',
+  \ '        "byte": 680',
+  \ '      }',
+  \ '    }',
+  \ '  },',
+  \ '  {',
+  \ '    "severity": "error",',
+  \ '    "summary": "Could not load plugin",',
+  \ '    "detail": "Plugin reinitialization required. Please run \"terraform\""',
+  \ '  }',
+  \ '  ]',
+  \ '}',
   \ ])