Replace run.sh with a faster, filterable, DRYer python replacement

[thepetertessier]

I made run_system_tests.py as a replacement for run.sh, with a few improvements:

• It executes tests in parallel by default. On my system, this decreased test execution time by about 90%, going from 42.151s to 4.364s.
• It has command-line parameters to filter tests (-k to filter by keyword and -d to filter by directory)
• Instead of printing a dot for a success, it prints which test passed. It also gives more meaningful output on a failure, including a summary of which tests failed at the end.
• It is much more modular. Adding the tests for the sip extension takes only a few lines of code, reusing the logic for selftests. Even with the extra features and comments, the whole file is only 350 lines (excluding the long documentation at the beginning and imports) compared to 300 lines of run.sh.

While developing this, I also found a small bug where executing tipc --pa=.../unwritable foo.tip exits with a success instead of a failure:

fpv5gr@portal06 ~/tipc/build $ touch unwritable
fpv5gr@portal06 ~/tipc/build $ chmod 444 unwritable
fpv5gr@portal06 ~/tipc/build $ ./src/tipc ../test/system/iotests/fib.tip --pa=unwritable
tipc: error: failed to open 'unwritable' for writing
fpv5gr@portal06 ~/tipc/build $ echo $?
0

So I fixed that too.