elf: Check that --list-diagnostics output has the expected syntax

Parts of elf/tst-rtld-list-diagnostics.py have been copied from
scripts/tst-ld-trace.py.

The abnf module is entirely optional and used to verify the
ABNF grammar as included in the manual.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>