Issue 3472: several fixes to input/regression/scheme-text-spanner.ly

Issue 3472: several fixes to input/regression/scheme-text-spanner.ly

* The file `input/regression/scheme-text-spanner.ly' assigns a constant
pair to the variable `event-drul' and subsequently attempts to modify
the pair.  This can lead to scheme-text-spanners not appearing when
requested in simultaneous contexts.  This patch corrects the faulty usage,
replacing `event-drul' with two variables `event-start' and `event-stop'.

* The variable `current-event' is not necessary, and has been removed.

* Tabs in the engraver definition have been replaced with spaces.

Signed-off-by: David Kastrup <dak@gnu.org>