test/run_tests.pl: Ensure that all HARNESS_VERBOSE values are respected

test/run_tests.pl: Ensure that all HARNESS_VERBOSE values are respected

... with perl truthiness in mind

Most of all, this means not having undue expectations that its value
is numerical (this is particularly true when HARNESS_VERBOSE isn't given
by the user, and this script's default is "yes").

We do this by ensuring that $tap_verbosity is turned into an appropriate
number if HARNESS_VERBOSE's value isn't numerical.

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29443)