Use set -ex instead of /bin/sh -ex

Simply because it is subjectively more typical and easier to see; use
`set -ex` instead of `-ex` on the shebang line.

Author: tcharding

contrib/test.sh

@@ -1,6 +1,6 @@
-#!/bin/sh -ex
+#!/bin/sh
 
-set -e
+set -ex
 
 # TODO: Add "alloc" once we bump MSRV to past 1.29
 FEATURES="bitcoin_hashes global-context lowmemory rand recovery serde std"