SystemTestMixin.set_up_grid configures an introducer that tries to automatically select a main tub port

[exarkun]

allmydata.test.common_system.SystemTestMixin._create_introducer does not supply a configuration value for tub.port or tub.location. Consequently, the introducer created by this function tries to pick a free TCP port on its own to use for the main tub. Since this process is subject to race conditions, this sometimes fails. Since _create_introducer is called by set_up_nodes and set_up_nodes is essentially the whole point of SystemTestMixin, all test cases that use SystemTestMixin are subject to this intermittent failure condition.

Instead, _create_introducer should supply a configuration that specifies a reliably available port for the main tub.

[exarkun]

Fixed in 5bfe6d5b24813c6ef0107f64f66dc4027917c813 but I got the commit message syntax wrong