patch 8.0.0822: Test_with_partial_callback is a tiny bit flaky

Problem:    Test_with_partial_callback is a tiny bit flaky.
Solution:   Add it to the list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index c57ec5f..b1d9d94 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -233,6 +233,7 @@
       \ 'Test_quoteplus()',
       \ 'Test_quotestar()',
       \ 'Test_reltime()',
+      \ 'Test_with_partial_callback()',
       \ ]
 
 " Locate Test_ functions and execute them.