Add fixer tidy for HTML

Add tests for tidy HTML fixer

test/fixers/test_tidy_fixer_callback.vader

@@ -0,0 +1,29 @@
+Before:
+  Save g:ale_html_tidy_executable
+
+  let g:ale_html_tidy_executable = 'tidy_paths/tidy'
+
+  call ale#test#SetDirectory('/testplugin/test/fixers')
+
+  silent cd ..
+  silent cd command_callback
+  let g:dir = getcwd()
+
+After:
+  Restore
+
+  call ale#test#RestoreDirectory()
+
+Execute(The tidy callback should return 0 if tidy not found):
+  let g:ale_html_tidy_executable = 'xxxinvalidpath'
+  AssertEqual
+  \ 0,
+  \ ale#fixers#tidy#Fix(bufnr(''))
+
+Execute(The tidy callback should return the correct default command):
+  AssertEqual
+  \ {
+  \   'command': ale#Escape('tidy_paths/tidy')
+  \     . ' -q --tidy-mark no --show-errors 0 --show-warnings 0'
+  \ },
+  \ ale#fixers#tidy#Fix(bufnr(''))