Add junit output to run_unit_tests.py

Add junit output to run_unit_tests.py.

Adding `--junit=output.xml` to the run_unit_tests.py command line will produce junit-style output in `output.xml`.

The root element for the xml output is `<testsuite>`.  Depending on Jenkins' requirements, we could add a `<testsuites>` element above that.  Each `testsuite` represents a test driver, and each `testcase` represents a numbered test case.  `stdout` output from the test driver is captured as normal xml text in a `<system-out>` element in each `testcase`.

Note that in order to include runtime and pass/fail statistics as attributes to the `<testsuite>` and `<testcase>` elements, the xml output has to be processed in memory and output all at once when all tests have been completed.  This means that very large test drivers (like `bslstl_string.t.cpp`) should not be run in `V=2` mode or above.