Set breakpoint condition-evaluation in forking-threads-plus-breakpoint.exp

Set breakpoint condition-evaluation in forking-threads-plus-breakpoint.exp

Proc do_test in forking-threads-plus-breakpoint.exp has an argument
cond_bp_target, but the test doesn't use it to set
"breakpoint condition-evaluation", which is an oversight in the test.

This patch fixes it by setting "breakpoint condition-evaluation" per
$cond_bp_target.

gdb/testsuite:

2016-02-18  Yao Qi  <yao.qi@linaro.org>

	* gdb.threads/forking-threads-plus-breakpoint.exp (do_test):
	Set "set breakpoint condition-evaluation" per $cond_bp_target.