SetDERPMap spawns a goroutine that calls ReSTUN, which logs via the
test logger. If the test returns before that goroutine logs, the
goroutine races with testing cleanup.
Use tstest.WhileTestRunningLogger so the goroutine's logf call becomes
a no-op once the test finishes.
Fixes#19829
Change-Id: I1097f98e40ffd1c5dd7fb7a715c918255853e3c6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>