defconfig: sdm855: Enable kernel self test modules for SDM855 SoC.

Enable RCU_TORTURE, LKDTM and ATOMIC64_SELFTEST test modules
for sdm855 soc for testing. These flags are required only on
defconfig for testing and not required on perf_defconfig.

Change-Id: Ic37b6e45bc49da9495266eb45adc98bbd4ec3d74
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>