Improve base time of assert_linear_performance (ruby#11369)

Remove `.ceil` from base time calculation that makes 10x gap.
This will make the assertion more strict and also less flaky.

tool/lib/core_assertions.rb

@@ -768,7 +768,9 @@ def assert_linear_performance(seq, rehearsal: nil, pre: ->(n) {n})
         end
         times.compact!
         tmin, tmax = times.minmax
-        tbase = 10 ** Math.log10(tmax * ([(tmax / tmin), 2].max ** 2)).ceil
+
+        # safe_factor * tmax * rehearsal_time_variance_factor(equals to 1 when variance is small)
+        tbase = 10 * tmax * [(tmax / tmin) ** 2 / 4, 1].max
         info = "(tmin: #{tmin}, tmax: #{tmax}, tbase: #{tbase})"
 
         seq.each do |i|