patch 8.0.0708: some tests are old style

Problem:    Some tests are old style.
Solution:   Change a few tests from old style to new style. (pschuh,
            closes #1813)

src/testdir/test_exists_autocmd.vim

@@ -0,0 +1,26 @@
+" Test that groups and patterns are tested correctly when calling exists() for
+" autocommands.
+
+function Test_AutoCommands()
+  let results=[]
+  augroup auexists
+  augroup END
+  call assert_true(exists("##BufEnter"))
+  call assert_false(exists("#BufEnter"))
+  au BufEnter * let g:entered=1
+  call assert_true(exists("#BufEnter"))
+  call assert_false(exists("#auexists#BufEnter"))
+  augroup auexists
+  au BufEnter * let g:entered=1
+  augroup END
+  call assert_true(exists("#auexists#BufEnter"))
+  call assert_false(exists("#BufEnter#*.test"))
+  au BufEnter *.test let g:entered=1
+  call assert_true(exists("#BufEnter#*.test"))
+  edit testfile.test
+  call assert_false(exists("#BufEnter#<buffer>"))
+  au BufEnter <buffer> let g:entered=1
+  call assert_true(exists("#BufEnter#<buffer>"))
+  edit testfile2.test
+  call assert_false(exists("#BufEnter#<buffer>"))
+endfunction