selftests/bpf: add tests for LIBBPF_BPF_TOKEN_PATH envvar

Add new subtest validating LIBBPF_BPF_TOKEN_PATH envvar semantics.
Extend existing test to validate that LIBBPF_BPF_TOKEN_PATH allows to
disable implicit BPF token creation by setting envvar to empty string.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231213190842.3844987-11-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>