timeout.test: fix test for systems that don't have bash.

(Such as Android.)
diff --git a/tests/timeout.test b/tests/timeout.test
index 386441a..4299403 100755
--- a/tests/timeout.test
+++ b/tests/timeout.test
@@ -22,7 +22,7 @@
 # There's another special case where if the subprocess catches our timeout
 # signal and exits, we need to report that as a timeout (unless overridden).
 cat > loop.sh <<EOF
-#!/bin/bash
+#!/bin/sh
 trap "exit 3" SIGTERM
 while true; do
   :