sapling: disable flaky tests on github getdeps cli

Summary:
X-link: facebook/sapling#973

X-link: facebookincubator/zstrong#1017

* disable flaky tests as per comment in Makefile
* fix Makefiles JOBS sed expression to be macOS make compatible
* remove Makefile comment in multiline shell, macOS make didn't like it
* remove hexdump from deps for macOS where it is a system util

Reviewed By: quark-zju

Differential Revision: D64781284

fbshipit-source-id: a7f52c67d430219dcf7173b8d03cafe32fe41989

build/fbcode_builder/manifests/sapling

@@ -60,7 +60,7 @@ fb303
 fbthrift
 rust-shed
 
-[dependencies.test=on]
+[dependencies.all(test=on,not(os=darwin))]
 hexdump
 
 [dependencies.not(os=windows)]