This removes use of gnutls and the TLS implementation, which is no
longer needed (task synchronization is handled via Taskchampion, which
uses `reqwest`, which handles TLS via other Rust dependencies). This
incidentally removes the following config options:
* `debug.tls`
* `taskd.ca`
* `taskd.certificate`
* `taskd.ciphers`
* `taskd.credentials`
* `taskd.key`
* `taskd.server`
* `taskd.trust`
This sets `set -e` to ensure any errors in the test script are
propagated upwards and noticed by the Github workflow.
Prior to these changes failures in the build process of the tests would
be ignored, and the success / failure would be determined purely by the
return code of the `./problems` script.
