Use sh instead of bash in tests.

tests/test-clone

@@ -1,4 +1,4 b''
-#!/bin/bash
+#!/bin/sh
 
 mkdir a
 cd a

tests/test-clone-failure

@@ -1,4 +1,4 b''
-#!/bin/bash
+#!/bin/sh
 
 # No local source
 hg clone a b