…ort.

* Access source files via {{ test_srcdir }} everywhere.

* Define TEST_RUNDIR, use it to access source files in shell scripts, instead of futzing with
  $TEST_SRCDIR/$TEST_WORKSPACE.

* Wrap sh_test/cc_tests with sh_test_wrapper.sh and provide the ability to override main for C++
  tests. This allows site specific customizations to the environment, e.g. overriding TEST_RUNDIR.

* Allow TestEnvironment::exec to support Python execution via symlinks.

* Some documentation in STYLE.md to capture the conventions.

* Fix a port dependency in admin_test.