[YDB#983] Test the new error message when running `mupip reorg -TRUNCATE` twice in a row.

Test that running mupip reorg -TRUNCATE twice in a row now puts out the more meaningful error message:

%GTM-I-MUTRUNCALREADY, Region DEFAULT: no further truncation possible

instead of the old

%YDB-I-MUTRUNCNOSPACE, Region DEFAULT has insufficient space to meet truncate target percentage of 0