Run "make clean" for test_120_smep_works in qrt_kernel_security

[Po-Hsu Lin]

When running the test_120_smep_works test in ubuntu_qrt_kernel_security, it will try to build the execuser.ko module with the header of the current running kernel.

For example,
if we test the AWS kernel first, the module will be:

$ modinfo execuser.ko
filename: /home/ubuntu/autotest/client/tmp/ubuntu_qrt_kernel_security/src/qa-regression-testing/scripts/kernel-security/smep/execuser/execuser.ko
license: GPL
srcversion: 1972E301DF4C7289B88349C
depends:
retpoline: Y
name: execuser
vermagic: 4.15.0-1021-aws SMP mod_unload

And then we reboot to the generic kernel and run it again, the module won't get rebuilt properly as the code didn't change.

This issue will cause the test to complain about invalid format.
Add "make clean" will make this test become more robust.