selftests/bpf: Add test for bpf_ima_file_hash()

Add new test to ensure that bpf_ima_file_hash() returns the digest of the
executed files.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220302111404.193900-6-roberto.sassu@huawei.com