Use -N for faster tests

http2 v4.1.0 disables flow control, this makes generator of infinite body
consume too much CPU and starve other threads, making tests slow. Giving more
threads to the test binary speeds the tests up quite significantly.

Upstream Issue: kazu-yamamoto/http2#62

libs/http2-manager/http2-manager.cabal

@@ -41,7 +41,7 @@ test-suite http2-manager-tests
   ghc-options:
     -Wall -Wincomplete-uni-patterns -Wincomplete-record-updates
     -Wpartial-fields -fwarn-tabs -optP-Wno-nonportable-include-path
-    -Wredundant-constraints -threaded
+    -Wredundant-constraints -threaded -with-rtsopts=-N
 
   -- cabal-fmt: expand test
   other-modules: