tests/tcg/meson.build: fix build_test_depends

build_test_depends is supposed to be an array, not a custom target. In
case we go through this path, we have an error with following lines:

      exe_depends = build_test_depends
      if 'depends' in setup
        exe_depends += setup['depends']
      endif

where meson complains that custom target has no += operator.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Link: https://lore.kernel.org/qemu-devel/20260904200614.2808877-1-pierrick.bouvier@oss.qualcomm.com
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>