ipn/ipnlocal: make TestStateMachine less flaky (#19434)

TestStateMachine & TestStateMachineSeamless both flake a lot asserting the
"Shutdown" call on cc after a Logout. This is because Shutdown is called on
a goroutine to avoid a deadlock if it's called while holding the
LocalBackend lock (#18052).

This fixes that cause of flakes by waiting for LocalBackend's goroutine
tracker to have no goroutines running (so the goroutine that calls Shutdown
must have finished).

This does not make TestStateMachine non-flaky because it can flake later in
the test, too: the assertion on "unpause" after clearing the netmap between
"Start4" and "Start4 -> netmap" sometimes fails.

Updates tailscale/corp#36230
Updates #19377
Updates #18052

Signed-off-by: James Sanderson <jsanderson@tailscale.com>

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