patch 8.2.0203: :helptags and some other functionality not tested

Problem:    :helptags and some other functionality not tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5567)
diff --git a/src/testdir/test_compiler.vim b/src/testdir/test_compiler.vim
index 1ce2718..4f02cf1 100644
--- a/src/testdir/test_compiler.vim
+++ b/src/testdir/test_compiler.vim
@@ -60,5 +60,9 @@
 endfunc
 
 func Test_compiler_error()
+  let g:current_compiler = 'abc'
   call assert_fails('compiler doesnotexist', 'E666:')
+  call assert_equal('abc', g:current_compiler)
+  call assert_fails('compiler! doesnotexist', 'E666:')
+  unlet! g:current_compiler
 endfunc