Update @variance testcases after fixing PR20599.

Update @variance testcases after fixing PR20599.

This update fixes the reference values for @variance where PR20599
(histogram breaks @variance) manifested.

For linear_large.exp and linear_large_neg.exp, remove @variance
tests because of arithmetic overflow.

linear.exp: Update the @variance reference value.
linear_nearlyempty.exp: Ditto.
linear_over.exp: Ditto.
linear_overunder.exp: Ditto.
linear_under.exp: Ditto.
linear_large.exp: Remove the @variance subtest.
linear_large.stp: Ditto.
linear_large_neg.exp: Ditto.
linear_large_neg.stp: Ditto