adding tests

test/CDB_StatsTest.sql

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+-- continuous uniform distribution has kurtosis = -6/5, skewness = 0.0
+-- http://mathworld.wolfram.com/UniformDistribution.html
+
+With dist As (
+  SELECT random() As val
+  FROM generate_series(1,5000000) t
+),
+m As (
+  SELECT avg(val) mn, count(*) cnt, stddev(val) s
+  FROM dist
+  )
+
+SELECT 
+  abs(sum(power(mn - val,4)) / ( cnt * power(s,4)) - 3 + 1.20) < 1e-3 As kurtosis,
+  abs(sum(power(mn - val,3)) / ( cnt * power(s,3))) < 1e-3 As skewness
+FROM dist, m
+GROUP BY m.cnt, m.mn, m.s

test/CDB_StatsTest_expect

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+true|true