pgsql: Make ssl certificate for ssl_passphrase_callback test via Makefi

Make ssl certificate for ssl_passphrase_callback test via Makefile

The recipe was previously given in comments in the module's test
script, but now we have an explicit recipe in the Makefile. The now
redundant comments in the script are removed.

This recipe shouldn't be needed in normal use, as the certificate and
key are in git and don't need to be regenerated.

Discussion: https://postgr.es/m/ae8f21fc-95cb-c98a-f241-1936133f466f@2ndQuadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b846091fd0a7a747933232016f0a52aa764398b8

Modified Files
--------------
src/test/modules/ssl_passphrase_callback/Makefile      | 18 ++++++++++++++++++
.../modules/ssl_passphrase_callback/t/001_testfunc.pl  |  7 +------
2 files changed, 19 insertions(+), 6 deletions(-)