test: move slow test from simple/ to pummel/

Move simple/test-http-many-keep-alive-connections to the pummel/
directory, it takes about 2.5 seconds to complete.

diff --git a/test/simple/test-http-many-keep-alive-connections.js b/test/pummel/test-http-many-keep-alive-connections.js
similarity index 100%
rename from test/simple/test-http-many-keep-alive-connections.js
rename to test/pummel/test-http-many-keep-alive-connections.js