Since Mailbox driver is same for TC0 and SGM, so renamed guide_sgm_testing.md...

Since Mailbox driver is same for TC0 and SGM, so renamed guide_sgm_testing.md into guide_linux_testing.md
Updated SGM reference also from coresponding files

Updated user_guide.md for the same

Signed-off-by: Monalisa Jena <monalisa.jena@arm.com>