run_tests.pl: Improve diagnostics on the use of HARNESS_JOBS
run_tests.pl: Improve diagnostics on the use of HARNESS_JOBS

Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13551)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl