patch 8.1.1544: some balloon tests don't run when they can

Problem:    Some balloon tests don't run when they can.
Solution:   Split GUI balloon tests off into a separate file. (Ozaki Kiichi,
            closes #4538)  Change the feature check into a command for
            consistency.
diff --git a/src/testdir/test_float_func.vim b/src/testdir/test_float_func.vim
index 6340b53..f407f40 100644
--- a/src/testdir/test_float_func.vim
+++ b/src/testdir/test_float_func.vim
@@ -1,8 +1,7 @@
 " test float functions
 
-if !has('float')
-  throw 'Skipped, float feature missing'
-end
+source check.vim
+CheckFeature float
 
 func Test_abs()
   call assert_equal('1.23', string(abs(1.23)))