Fix certificate paths to use perl2host
Commit
c113d8ad50 moved the copying of certificates into a temporary path
for the duration of the tests, instead of using the source tree. This broke
the tests on msys as the absolute path wasn't adapted for the msys platform.
Ensure to convert the path with perl2host before copying and passing in the
connection string.
While there also make certificate copying error handling uniform across all
the test suites.
Discussion: https://postgr.es/m/YacT3tm97xziSUFw@paquier.xyz