selftest/bpf: Verifier tests for var-off access

Add tests for the new functionality - reading and writing to the stack
through a variable-offset pointer.

Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210207011027.676572-4-andreimatei1@gmail.com