Move `call_until_true` to tempest/lib

This `call_until_true()` is handy and could be used in Tempest plugins.
Let's move it to tempest/lib.

Also add some unit tests.

Change-Id: Ie379030baa336239e6027c8f3cdbeb74c561f66b
diff --git a/requirements.txt b/requirements.txt
index d698cda..a773d16 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -22,3 +22,4 @@
 PrettyTable<0.8,>=0.7 # BSD
 os-testr>=0.7.0 # Apache-2.0
 urllib3>=1.15.1 # MIT
+debtcollector>=1.2.0 # Apache-2.0