core_assertions.rb: Refine `assert_linear_performance`

* Calculate each timeout from the ratio of each factor to the first
  factor.

tool/lib/core_assertions.rb

@@ -760,7 +760,7 @@ eom
 
         seq.each do |i|
           next if i == first
-          t = (tmax * i).to_f
+          t = tmax * i.fdiv(first)
           *arg = pre.call(i)
           message = "[#{i}]: in #{t}s"
           Timeout.timeout(t, nil, message) do