Fix undeclared variable when test_suite parameter is missing.

SlapOS Master doesn't store any "news" about slave instances. Assume true.

Noticed by Cédric de Saint Martin.

Launch one tester per test.

Because sale trade condtion is already applied when it is set.

Suggested by Cédric de Saint Martin <cedric.dsm@tiolive.com>

xpath expression is modified, <tbody> is add to the filter
expression in order to do not follow any <tr> found inside <thead>.

This reverts commit 00475f4f.