Parallel tests: try a new port if they chosen port is busy

T:builtin:1:A
A:builtin:System test builtin
I:builtin:PORTRANGE:5900 - 5999
testsock.pl: bind(10.53.0.1, 5900): Address already in use
I:builtin:Network interface aliases not set up.  Skipping test.
R:builtin:UNTESTED
E:builtin:Wed 18 Apr 2018 08:32:01 AEST