patch 8.1.1300: in a terminal 'ballooneval' does not work right away

Problem:    In a terminal 'ballooneval' does not work right away.
Solution:   Flush output after drawing the balloon.  Add the <Ignore> key
            code.  Add a test.
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 2bda742..6b966e6 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -68,6 +68,7 @@
 	test_autoload \
 	test_backspace_opt \
 	test_backup \
+	test_balloon \
 	test_behave \
 	test_blob \
 	test_blockedit \
@@ -294,6 +295,7 @@
 	test_autocmd.res \
 	test_autoload.res \
 	test_backspace_opt.res \
+	test_balloon.res \
 	test_blob.res \
 	test_blockedit.res \
 	test_breakindent.res \