This patch fixes a data race in wgengine/netstack that surfaced while
running both TestTCPForwardLimits and TestTCPForwardLimits_PerClient.
Because these two tests both setup the TS_DEBUG_NETSTACK envknob, a
race happens because netstack.Impl.Close leaked its inject goroutine.
The inject goroutine also reads the TS_DEBUG_NETSTACK envknob, so if
it is still running when the next test starts, then it will break.
This patch also cleans up the tests a bit, ensuring that neither of
them run in T.Parallel. It also adds a T.Cleanup call to clear the
envknob.
Fixes#19720
Signed-off-by: Simon Law <sfllaw@tailscale.com>
Simon Law
e4e59a2af0