Update pltcl regress test's expected output for 7.0 psql output format.

Update pltcl regress test's expected output for 7.0 psql output format.
NOTE: this implementation of tcl_avg() fails with 'divide by zero'
for zero input rows.  It ought to return NULL, but pltcl does not
currently provide a way to do that, so I'm leaving the problem unsolved
for now.