tests: gstinfo: Make logging tests pass when GST_DEBUG is set

tests: gstinfo: Make logging tests pass when GST_DEBUG is set

Use gst_debug_set_threshold_from_string's new reset behavior to undo
GST_DEBUG and ensure the logging tests have a known configuration.

`gst_debug_set_threshold_from_string ("LOG", TRUE)` has the same effect
as `gst_debug_set_threshold_from_string ("", TRUE)` followed by
`gst_debug_set_default_threshold (GST_LEVEL_LOG)`.

Don't bother remembering the default log level set when the test
started. It will get reset by the next test, anyway.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/605>