Remove bashisms from tests.

Those tests don't actually need bash. Just use common shebang.

tests/test_sha512sum.in

@@ -1,4 +1,4 @@
-#! /bin/bash
+#! /bin/sh
 
 # create a randome file
 file="`mktemp "${TMPDIR:-/tmp}/tmp.XXXXXXXXXX"`" || exit 1