fix(tests): fix ale_c_build_dir_names being unset in tests (#5109)

- Use ale#Set() to set the ale_c_build_dir_names variable.
- Ensure SetUpLinterTest() is called before any Save commands in tests.
- Add c.vim to runtime before non-linter tests are executed.
- Remove workarounds in c.vim.

test/linter/test_cpp_cc.vader

@@ -1,4 +1,6 @@
 Before:
+  call ale#assert#SetUpLinterTest('cpp', 'cc')
+
   Save g:ale_c_parse_makefile
   Save g:ale_history_enabled
 
@@ -8,7 +10,6 @@ Before:
   let g:get_cflags_return_value = ''
   let g:executable_map = {}
 
-  runtime autoload/ale/c.vim
   runtime autoload/ale/engine.vim
 
   function! ale#engine#IsExecutable(buffer, executable) abort
@@ -19,8 +20,6 @@ Before:
     return g:get_cflags_return_value
   endfunction
 
-  call ale#assert#SetUpLinterTest('cpp', 'cc')
-
   let b:command_tail = ' -S -x c++'
   \   . ' -o ' . (has('win32') ? 'nul': '/dev/null')
   \   . ' -iquote %s:h'