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_perl.vim b/src/testdir/test_perl.vim
index d38fc07..8f23f2c 100644
--- a/src/testdir/test_perl.vim
+++ b/src/testdir/test_perl.vim
@@ -1,8 +1,7 @@
 " Tests for Perl interface
 
-if !has('perl')
-  throw 'Skipped, perl feature missing'
-end
+source check.vim
+CheckFeature perl
 
 " FIXME: RunTest don't see any error when Perl abort...
 perl $SIG{__WARN__} = sub { die "Unexpected warnings from perl: @_" };