tests: Use /bin/sh shebang

Bashisms for tests were removed in ecd44e68 ("tests: Remove
bashisms (s/source/.)"), so no need to use bash shebang.

+ remove trailing whitespace.

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>

testsuite/tests/tc/dsmark.t

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 # vim: ft=sh
 
 . lib/generic.sh