bpf/selftests: Add selftests for new task kfuncs

A previous change added a series of kfuncs for storing struct
task_struct objects as referenced kptrs. This patch adds a new
task_kfunc test suite for validating their expected behavior.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221120051004.3605026-5-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>