Use test-specific temp path for keys during SSL test

Use test-specific temp path for keys during SSL test

The SSL and SCRAM TAP test suites both use temporary copies of the
supplied test keys in order to ensure correct permissions.  These
were however copied inside the tree using temporary filenames rather
than a true temporary folder.  Fix by using tmp_check supplied by
PostgreSQL::Test::Utils. Spotted by Tom Lane during review of the
nearby sslinfo TAP test patch.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/599244.1638041239@sss.pgh.pa.us