A connection handler for the package's tests and vignettes, built from
environment variables each time it is used:
SIGREPO_TEST_DB_USER and SIGREPO_TEST_DB_PASSWORD (required,
no defaults), and optionally SIGREPO_TEST_DB_NAME,
SIGREPO_TEST_DB_HOST, SIGREPO_TEST_DB_PORT,
SIGREPO_TEST_API_HOST and SIGREPO_TEST_API_PORT, which default
to sigrepo.org. When the user or password is unset, user and
password are empty, the database tests skip, and the vignettes show
their code without running it.