Add -Wno-error=shadow build option for kdbus tests

Add -Wno-error=shadow build option for kdbus tests

In some different build environment (e.g., product divisions),
several strict compilation options can be added like -Werror=shadow.
Shadowing is intentional and not a bug in tests so that we avoid this build error
by adding -Wno-error=shadow

[  105s] kdbus-test.h:42:20: error: declaration of '_ASSERT_RETURN_VAL_val0_' shadows
a previous local [-Werror=shadow]
[  105s]    42 |  __auto_type const _ASSERT_RETURN_VAL_val0_ = (val0);\

Change-Id: Ia4f8a746dce0b39c896daf2e4cd1c87f62a8dadd
Signed-off-by: Hyotaek Shim <hyotaek.shim@samsung.com>