selftests/bpf: Add remaining ASSERT_xxx() variants

Add ASSERT_TRUE/ASSERT_FALSE for conditions calculated with custom logic to
true/false. Also add remaining arithmetical assertions:
  - ASSERT_LE -- less than or equal;
  - ASSERT_GT -- greater than;
  - ASSERT_GE -- greater than or equal.
This should cover most scenarios where people fall back to error-prone
CHECK()s.

Also extend ASSERT_ERR() to print out errno, in addition to direct error.

Also convert few CHECK() instances to ensure new ASSERT_xxx() variants work as
expected. Subsequent patch will also use ASSERT_TRUE/ASSERT_FALSE more
extensively.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Lorenz Bauer <lmb@cloudflare.com>
Link: https://lore.kernel.org/bpf/20210426192949.416837-2-andrii@kernel.org