testing: do not require telepathy

testing: do not require telepathy

Previously, all testing was disabled when the Telepathy backend was
disabled. That is too strict. For example, the EDS backend tests can
work without it.

Only the core folks tests and of course the Telepathy tests depend on
the Telepathy backend and thus must be disabled when that backend is
disabled.

Adding the TP backend path to the FOLKS_BACKEND_PATH env variable
must be conditional, otherwise there are errors about not finding that
.so file when running the other tests.