testnode: run first found runTestSuite from lowest matching partition, not random one.

glob uses os.listdir, which returns list in arbitrary order.
Sometimes we have seevral partitions containing bin/runTestSuite. We want to use the one closest to the root instance (i.e alphabetical order, test0-1/bin/runTestSuite is better thantest0-3/bin/runTestSuite).

Usecase: webrunner, for which main instance contains a bin/runTestSuite for unit tests. But resiliency tests (dedicated partition, usually test0-0) also has bin/runTestSuite, and then requests a normal instance of slaprunner (usually test0-3).