add "ok=sigpipe" to test_must_fail and use it to fix flaky tests

diff --git a/RelNotes b/RelNotes
index 58c32c35250bcdeb46b0d6340ec3d246a5e6298c..3ba13ce25351c60c93d0c8ed78f207d9a591d517 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes/1.9.2.txt
\ No newline at end of file
+Documentation/RelNotes/2.7.0.txt
\ No newline at end of file