tests: increase timeout on testCPUSecurityMitigationsEnable

This test barely makes it within the default 10 minutes timeout.
From what I see most of the time is spent by waiting for multiple
reboots of the machine.
Locally this took almost 7 minutes to run so for CI we can bump this
timeout to 20 minutes.

test/verify/check-system-info

@@ -511,6 +511,7 @@ machine : 8561
  spoof_threads(2, expect_link_present=True, expect_smt_state=self.expect_smt_default, cmdline=None)
 
  @testlib.skipImage("TODO: add Arch Linux grub entry support", "arch")
+ @testlib.timeout(1200)
  def testCPUSecurityMitigationsEnable(self):
  b = self.browser
  m = self.machine