tests: remove EnsureLoaded

This should not be necessary in general, and causes e.g. :FZF to be
loaded from the global Vim runtime (causing an assertion failure in the
tests).

It is replaced by a verbatim / trimmed down `:runtime` for the "Check
installed plugins" test.

test/test.vader

@@ -44,12 +44,6 @@ Execute (Initialize test environment):
   endfunction
   command! -nargs=+ -bang AssertExpect call AssertExpect('<bang>' == '!', <args>)
 
-  function! EnsureLoaded()
-    if has('vim_starting')
-      runtime! plugin/**/*.vim
-    endif
-  endfunction
-
   function! RmRf(file)
     call system(printf('rm -rf "%s"', a:file))
   endfunction
@@ -98,7 +92,6 @@ Execute (Cleanup):
   silent! delf PlugStatusSorted
   silent! delf AssertExpect
   silent! delf PlugUpdated
-  silent! delf EnsureLoaded
   silent! delf ReloadPlug
   silent! delc AssertExpect
   silent! unmap /