wgengine/magicsock: minor tidies in Test_endpoint_maybeProbeUDPLifetimeLocked

* Remove a couple of single-letter `l` variables
* Use named struct parameters in the test cases for readability
* Delete `wantAfterInactivityForFn` parameter when it returns the
  default zero

Updates #cleanup

Signed-off-by: Alex Chan <alexc@tailscale.com>
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