patch 8.0.0401: test fails with missing balloon feature

Problem:    Test fails with missing balloon feature.
Solution:   Add check for balloon feature.
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index eeeb4e3..cd58f63 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -468,6 +468,8 @@
 endfunc
 
 func Test_balloon_show()
-  " This won't do anything but must not crash either.
-  call balloon_show('hi!')
+  if has('balloon_eval')
+    " This won't do anything but must not crash either.
+    call balloon_show('hi!')
+  endif
 endfunc