cmd/containerboot: avoid leaking bash scripts after test runs

The test was sending SIGKILL to containerboot, which results in no
signal propagation down to the bash script that is running as a child
process, thus it leaks.

Minor changes to the test daemon script, so that it cleans up the socket
that it creates on exit, and spawns fewer processes.

Fixes tailscale/corp#14833
Signed-off-by: James Tucker <james@tailscale.com>

cmd/containerboot/main_test.go

@@ -689,7 +689,7 @@ func TestContainerBoot(t *testing.T) {
 				t.Fatalf("starting containerboot: %v", err)
 			}
 			defer func() {
-				cmd.Process.Signal(unix.SIGKILL)
+				cmd.Process.Signal(unix.SIGTERM)
 				cmd.Process.Wait()
 			}()