patch 7.4.1479

Problem:    No testfor ":loadplugin".
Solution:   Add a test.  Fix how option is being set.

src/testdir/test_loadplugin.vim

@@ -0,0 +1,25 @@
+" Tests for :loadplugin
+
+func Test_loadplugin()
+  let topdir = expand('%:h') . '/Xdir'
+  exe 'set packpath=' . topdir
+  let plugdir = topdir . '/pack/mine/opt/mytest'
+  call mkdir(plugdir . '/plugin', 'p')
+  call mkdir(plugdir . '/ftdetect', 'p')
+  filetype on
+  try
+    exe 'split ' . plugdir . '/plugin/test.vim'
+    call setline(1, 'let g:plugin_works = 42')
+    wq
+
+    exe 'split ' . plugdir . '/ftdetect/test.vim'
+    call setline(1, 'let g:ftdetect_works = 17')
+    wq
+
+    loadplugin mytest
+    call assert_true(42, g:plugin_works)
+    call assert_true(17, g:ftdetect_works)
+  finally
+    call delete(topdir, 'rf')
+  endtry
+endfunc