test: fix 20-test_dgst.t to use hexkey

test: fix 20-test_dgst.t to use hexkey

Currently 20-test_dgst.t calls a quite bogus command:

    $ openssl dgst -sha256 -hmac -macopt hexkey:FFFF test/data.bin test/data.bin
    hexkey:FFFF: No such file or directory
    HMAC-SHA2-256(test/data.bin)= b6727b7bb251dfa65846e0a8223bdd57d244aa6d7e312cb906d8e21f2dee3a57
    HMAC-SHA2-256(test/data.bin)= b6727b7bb251dfa65846e0a8223bdd57d244aa6d7e312cb906d8e21f2dee3a57
    805B632D4A730000:error:80000002:system library:file_ctrl:No such file or directory:crypto/bio/bss_file.c:297:calling fopen(hexkey:FFF, r)
    805B632D4A730000:error:10080002:BIO routines:file_ctrl:system lib:crypto/bio/bss_file.c:300:

Does not check status code, discards stderr, and verifies the
checksums as per above. Note that the checksum is for the HMAC key
"-macopt", and `hexkey:FFFF` is attempted to be opened as a file.

See HMAC values for key `-macopt` and `hexkey:FFFF` using `openssl-mac`:

    $ openssl mac -digest SHA256 -macopt hexkey:$(printf '%s' '-macopt' | xxd -p -u) -in ./test/data.bin HMAC
    B6727B7BB251DFA65846E0A8223BDD57D244AA6D7E312CB906D8E21F2DEE3A57

    $ openssl mac -digest SHA256 -macopt hexkey:FFFF -in ./test/data.bin HMAC
    7C02D4A17D2560A5BB6763EDBF33F3A34F415398F8F2E07F04B83FFD7C087DAE

Fix this test case to actually use HMAC with hexkey:FFFF as intended.

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@surgut.co.uk>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/24068)