testbot/PatchUtils: Add partial support for PARENTSRC in test directories.
The PARENTSRC may have extra tests not used by the current test directory. The right fix would be to analyze the makefile's SOURCES or C_SRCS lists but that information is not available to the TestBot server. So instead use the parent test list only if it has a single test since in that case we can reasonably infer that PARENTSRC is used to import that one test. This is sufficient to handle 66% of the cases.