testr: Use waf testonly and create a custom directory for new workers.

Using waf testonly helps speed up test runs, and the custom directory is
required to run multiple workers in parallel.

Change-Id: I339420c5c503c270a04888a1c990836cb0c91e94
Signed-Off-By: Jelmer Vernooij <[email protected]>
Reviewed-by: Andrew Bartlett <[email protected]>

Author: jelmer

.testr.conf

@@ -1,4 +1,4 @@
 [DEFAULT]
-test_command=./buildtools/bin/waf test --filtered-subunit $IDOPTION $LISTOPT
+test_command=./buildtools/bin/waf testonly --filtered-subunit $IDOPTION $LISTOPT --with-selftest-prefix=$(mktemp -d)
 test_id_option=--load-list $IDFILE
 test_list_option=--list