Call stack.Close on stacks when we are done with them in tcp_test. This avoids
leaking resources and reduces the test's flakiness when race/gotsan is enabled.
It also provides test coverage for the race also fixed in this change, which
can be reliably triggered with the stack.Close change (and without the other
changes) when race/gotsan is enabled.
The race was possible when calling Abort (via stack.Close) on an endpoint
processing a SYN segment as part of a passive connect.
Updates #1564
PiperOrigin-RevId: 297685432
c6bdc6b05b