logictest: deflake a test

We've recently merged a test that in very rare circumstances could
produce a float result that differs from the expected one by 1 in the
15th significant digit (after rounding). I believe that could occur,
e.g. when the 15th and 16th significant digits were `35`, and we matched
the spec of supporting 15 significant digits for floats, yet the
rounding makes us return an unexpected result. This commit rounds to the
precision of 1 digit less which should make the test non-flaky.

Release note: None

pkg/sql/logictest/testdata/logic_test/aggregate

@@ -858,10 +858,10 @@ VALUES (1, 1.1, 1.1),
        (18, 11.2, 11.2);
 
 query FRF
-select sqrdiff(i), round(sqrdiff(f), 13), sqrdiff(d)
+select sqrdiff(i), round(sqrdiff(f), 12), sqrdiff(d)
 from ifd
 ----
-206.8333333333333333333334 86.2483333333333 86.24833333333333333333333
+206.8333333333333333333334 86.248333333333 86.24833333333333333333333
 
 query FRF
 SELECT sqrdiff(i), round(sqrdiff(f), 2), sqrdiff(d)